2025+中国专家共识：联合应用抗病毒药物治疗流感（英文版）

Consensus

DOI:10.5582/bst.2025.01292

# Chinese expert consensus on the combined use of antiviral drugs for influenza

Jing Yuan1,8, Haimei Zhang1,8, Rongrong Zou1,8, Weizhen Wengl, Honglian Bai2, Jun Chen3,Yutian Chong4,Xilong Deng5,Liang Fu1,Yingyun Fu6,Wenjun Gao7,Zuojiong Gong8,Chenghui Huang9, He Huang10,Jia Huang1,Jiang1,Janning Jiang1,Rongmeng Jiang12,Yongfang Jiang13,Linghua Li5,Bingliang Lin4,Xi Liu14,Feng Lin15,Jie Peng16,Qingsong Qi17 Bo Qin18,Zhijun Qu19,Fei Shi1,Gang Wu20,Tao Wu21,Hui Wang22,Lingwei Wang7,Yan Wang23,Fei Xiao14,Feng Xu24,Juan Xu17,Kaijin Xu25,Haipeng Zhu17,Guanlin Zhou26,Guangbin Chen1,*,Hongzhou Lu1,*, National Medical Research Center for Infectious Diseases

'The Third People's Hospital of Shenzhen, Shenzhen, Guangdong, China;

2Foshan First People's Hospital, Foshan Hospital Affiliated to Southern University of Science and Technology, Foshan, Guangdong,China;

Shanghai Public Health Clinical Center, Fudan University,China;

4The Third Affiliated Hospital of Sun Yat-sen University,Guangzhou,Guangdong,China;

$^{5}$ Guangzhou Eighth People's Hospital, Guangzhou Medical University, Guangzhou,China;

6Shenzhen People's Hospital, Shenzhen, Guangdong, China;

7Zhongshan Second People's Hospital, Zhongshan, Guangdong, China;

8Renmin Hospital of Wuhan University, Wuhan, Hubei, China;

9Shenzhen Bao'an People's Hospital, Shenzhen, Guangdong, China;

10Shenzhen Second People's Hospital, Shenzhen, Guangdong, China;

11The First Affiliated Hospital of Guangxi Medical University, Nanning,Guangxi,China;

12 Beijing Ditan Hospital, Capital Medical University, Beijing, China;

13The Second Xiangya Hospital of Central South University,Changsha,Hunan,China;

14The Fifth Affiliated Hospital, Sun Yat-sen University, Zhuhai, Guangdong, China;

15Hainan General Hospital, Haikou, Hainan, China;

16Nanfang hospital, Southern Medical University,Guangzhou,Guangdong,China;

17The Ninth People's Hospital of Dongguan, Dongguan,Guangdong,China;

18The First Affiliated Hospital of Chongqing Medical University,Chongqing,China;

19Longgang Central Hospital of Shenzhen, Shenzhen, Guangdong, China;

20The Affiliated Hospital of Southwest Medical University, Luzhou, Sichuan, China;

21Hainan Public Health Clinical Center, Haikou, Hainan, China;

22Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China;

23Peking University First Hospital, Beijing, China;

24The Second Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang, China;

25The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, Zhejiang, China;

26The Fifth People's Hospital, Ganzhou, Ganzhou, Jiangxi, China.

SUMMARY: Influenza is an acute respiratory infectious disease caused by influenza viruses, and it poses a serious threat to global public health. High-risk groups include the elderly, infants and young children, pregnant women, and patients with chronic underlying diseases. These groups are prone to developing severe illness after infection, which can lead to serious complications and even death. Early antiviral treatment is key to reducing the rate of severe illness and death. Currently, authoritative guidelines at home and abroad recommend early, single-agent antiviral therapy as the standard regimen. However, anti-influenza virusotherapy has problems such as drug resistance and poor therapeutic effect. To address these problems, this consensus was developed by organizing experts from the departments of Infectious Diseases, Respiratory Medicine, Critical Care Medicine, and Pharmacy. These experts systematically sorted out domestic and international evidence on combined antiviral therapy for influenza and formulated expert recommendations on combined antiviral therapy for influenza in specific populations.

Keywords: influenza viruses,influenza antiviral therapy,resistance, combined therapy

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1. Introduction

Influenza viruses cause seasonal influenza and influenza pandemics, posing a serious threat to human health and public health. According to the 2017 Global Burden of Disease Study(GBD 2017), up to 145,000 people worldwide die each year due to influenza-associated lower respiratory tract infection(LRTI)(I). A study showed that during the period 2010-2015, there were an average of 88,100 influenza-related respiratory disease deaths per year in mainland China, equivalent to 8.2%of all respiratory disease deaths(2). Dr Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization(WHO), has warned that the threat of a pandemic flu remains a constant concern. Risk of new influenza viruses crossing from animals to humans and causing a real pandemic is ongoing. We must remain vigilant and be well-prepared(3).

Antiviral drugs play a crucial role in controlling influenza outbreaks and epidemics. Up to now, the primary antiviral medications approved for treating influenza virus infections include three classes:transmembrane protein M2 ion channel inhibitors,neuraminidase inhibitors(NAIs), and RNA-dependent RNA polymerase(RdRp) inhibitors(4). Recently, new anti-influenza virus drugs such as ZSP1273 have been launched. Meanwhile, more anti-influenza virus drugs with new targets and completely new mechanisms of action are under development. However, the types of currently available antiviral drugs for influenza remain relatively limited. Moreover, influenza viruses are highly prone to mutations, and the number of drug-resistant virus strains is constantly increasing. Drug resistance caused by viral mutations and drug abuse remains a serious issue, and monotherapy for influenza viruses is facing the challenge of drug resistance. For instance, the detection rate of neuraminidase inhibitor-resistant strains is on the rise among immunocompromised patients. Typical cases include the H275Y mutation in Influenza A virus(IAV)subtype H1N1 and the I221T/V mutation in Influenza B virus(IBV). In addition, the polymerase inhibitor baloxavir can induce the PA/I38T mutation. Furthermore,antiviral treatment of severe influenza still faces many challenges and uncertainties at present. According to relevant data, the diagnosis time of patients with severe influenza in China is relatively late, which may lead to delays in treatment timing. As a result, most patients miss the optimal time window for antiviral treatment(5).Patients with severe influenza may experience prolonged replication and shedding of the virus in the upper and lower respiratory tracts. The virus excretion time of severe patients is prolonged, and the duration of antiviral treatment may need to be extended(6,7).

Combination drug therapy has emerged as a key strategy to address drug resistance and enhance therapeutic efficacy in severe influenza cases. By leveraging synergistic effects to inhibit viral replication

through multiple targets, this approach offers distinct clinical advantages(8). Specifically, combination drug therapy can reduce emergence of drug-resistant viral strains and mitigate treatment-related adverse effects,which may in turn lower incidence of severe influenza and improve the success rate of treating severe cases.Han J et al(9) pointed out a critical issue: currently circulating IAV strains(such as H1N1 and H3N2) have developed resistance to neuraminidase inhibitors. What is worse, they are almost completely resistant to M2 ion channel inhibitors. This growing resistance problem is further exacerbated by the use of subtherapeutic doses in both clinical treatment and chemoprophylaxis. Novel therapies targeting host components and new strategies for combination therapy show potential for maximizing the reduction of viral resistance.

Currently, anti-influenza virus therapy recommended in national and international expert consensus statements and clinical practice guidelines is typically based on monotherapy. However, in specific clinical scenarios(such as severe infections, patients at risk of drug resistance, or patients with immunosuppression),combination therapy strategies should be considered. This approach is also expected to become one of the future development trends in influenza treatment. By using drugs that act on different targets, we can not only reduce the development of viral drug resistance and minimize adverse reactions caused by the dosage of a single drug but also formulate individualized treatment plans based on the severity of the patient's condition. Especially for patients with severe influenza, special attention should be paid to host immune regulation therapy(10).Therefore,the combination regimens proposed in this consensus are expert recommendations. They apply to populations that are critically ill, immunosuppressed, or suspected of having drug resistance.

These recommendations are intended to inform clinical decision-making, and they are not routine first-line recommendations. Any off-guideline medication must undergo individualized risk-benefit assessment and be fully communicated with the patient or their family.

2. Methods

To ensure this consensus has a solid evidence-based foundation, we conducted systematic searches in multiple well-known medical databases(including PubMed and Web of Science) by September 25, 2025,using the search formula"Combination Therapy" AND"Influenza Virus". We aimed to collect key studies in all relevant fields through comprehensive literature searches, to ensure this consensus was developed based on the best available evidence. The evidence-based medicine(EBM) evidence of this expert consensus adopts the 2011 Oxford Centre for Evidence-Based Medicine(OCEBM) Levels of Evidence and Grades of Recommendations(11)(Table 1).

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Table 1. Level of evidence

|Recommendation Strength Evidence Level|Description|Description| |---|---|---| |A|1a|System review of randomized controlled trials (RCTs))| ||1b|RCTs with small confidence intervals for results| ||1c|Any evidence showing an "all or nothing effect"| |B|2a|Systematic evaluation of cohort studies| ||2b|Individual cohort studies (including low-quality RCTs, e.g., those with >20% loss-to-follow-up rates)| ||2c|Studies based on patient outcomes| ||3a|Systematic evaluation of case-control studies| ||3b|Single case-control study| |C|4|Case series reports, low-quality cohort studies and low-quality case-control studies| |D|5|Expert opinion (i.e., speculation based solely on basic research or clinical experience that is not supported by clinical studies)|

3. Current primary antiviral drugs for influenza

3.1. Life cycle of influenza viruses and targets and mechanisms of antiviral drug action

The replication process of influenza virus comprises six core steps, including viral entry, viral uncoating.viral genome replication and transcription, viral protein translation, viral assembly, and viral budding(12). This series of highly ordered steps provides clear targets for antiviral drug development. First, during the viral invasion stage, hemagglutinin(HA) inhibitors block fusion of the virus with the host cell membrane to prevent infection. After virus entry, during the uncoating stage, M2 ion channel inhibitors and HA inhibitors suppress acidification inside the virus to prevent the release of the virus's genetic material.Subsequently, in the core process of viral genome replication and transcription in the cell nucleus, RdRp inhibitors can directly inhibit the replication of viral genetic information. RdRp inhibitors include three categories: RNA polymerase acidic protein inhibitors(PA),RNA polymerase basic protein 1 inhibitors(PB1),and RNA polymerase basic protein 2 inhibitors(PB2).When newly formed vRNPs need to be transported from the nucleus to the cytoplasm for assembly, vRNP export inhibitors can interrupt this process. During the viral assembly stage, NAIs, M2 ion channel inhibitors,and HA maturation inhibitors interfere with the correct processing of viral proteins. Finally, when progeny virus particles bud on the cell surface, NAIs prevent the virus from detaching from the host cell surface(Figure 1).

3.2. Anti-influenza virus treatment drugs

The current major anti-influenza virus therapies are listed in Table 2.

4. The necessity of combination antiviral therapy

4.1. Antiviral drug resistance in influenza viruses

Influenza virus is a pathogen with rapid mutation ability, and its genome can evolve through multiple mechanisms such as point mutations(such as variations in PB2, PA, and NA genes), segmental recombination and genomic recombination. At present, multiple key drug resistance sites have been identified: the S31N mutation of the M2 protein confers resistance to amantadine drugs; the H274Y and R292K mutations in the NA gene significantly reduce sensitivity to neuraminidase inhibitors(e.g., oseltamivir); and the 138T mutation in the PA protein significantly diminishes the antiviral activity of baloxavir. In addition, novel mutations such as the K229R mutation in the PB1 gene and the P653L mutation in the PA gene also indicate a potential risk of resistance to favipiravir(13).

Between 2018 and 2020, the resistance rate of Influenza A(H1N1)pdm09 strains to NAIs reached 1.3% worldwide. Meanwhile, IBV also exhibited a resistance rate of approximately 1%(14). Children,patients receiving prophylactic drug treatment, and individuals with impaired immune function(such as hematopoietic stem cell transplant recipients or immunocompromised patients) have become high-risk groups for NAI resistance(15-17). More notably,among the more than 30 newly identified drug-resistant mutations from 2016 to 2024, approximately 80% are distributed in IBV.These mutations can lead to a drastic reduction in drug sensitivity: a single mutation can reduce the inhibitory effect by 10 to 1,000 times, while multi-site synergistic mutations(such as the combination of H274Y and I222R) can even increase drug resistance by more than 10,000 times. In addition, some mutation combinations can also cause cross-resistance or multidrug resistance phenotypes.Particularly, after some drug-resistant strains acquire compensatory mutations(such as the H274Y variant accompanied by D354G), their replication fitness and transmission ability are restored.

Drug resistance of influenza viruses to antiviral agents has become a major challenge in clinical practice.Faced with the constantly evolving drug resistance of influenza viruses, there is an urgent need to optimize

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 Figure 1. Replication cycle of influenza virus and crucial steps targeted by virus-directed antiviral compounds. Replication cycle of influenza virus encompassing six core steps(viral entry, uncoating, replication and transcription, protein translation, assembly, and budding) and the key stages targeted by virus-directed antiviral compounds. Approved drugs for influenza treatment are indicated in bold.(Figure created with MedPeer)

Table 2. The main anti-influenza virus treatment drugs

|Category|Mechanism|Representative drugs| |---|---|---| |M2 ion channel inhibitors|Inhibits M2 ion channel function and interferes with viral capsidization.|Amantadine, Rimantadine| |Neuraminidase inhibitors|Inhibits neuraminidase activity and blocks virus budding|Oseltamivir, Zanamivir, Peramivir, Lanaminivir| |RNA polymerase acid protein inhibitors|Inhibits viral RNA polymerase activity and prevents viral synthesis|Baloxidavir marboxil, Baloxavir, Suraxavir| |RNA polymerase basic protein 1 inhibitors|Targeting the catalytic function of PB1 to block RNA chain synthesis protein 1 inhibitors|Favipiravir, Ribavirin| |RNA polymerase basic protein 2 inhibitors|Targeting the cap-binding domain of PB2 and blocking the transcription of viral mRNAs|ZSP1273, Pimodivir| |Hemagglutinin inhibitors|Prevents viral release by selectively inhibiting steps such as HA maturation, intracellular trafficking, and embedding in the host cell membrane|Arbidol, Monoclonal antibodies(CR6261, VIS410,MHAA4549A,MEDI8852,CT-P27)|

existing influenza antiviral treatment strategies,explore combined treatment regimens, and develop drugs targeting novel targets.These measures will help effectively address the challenges posed by influenza virus drug resistance.

<BeginOfPara>Recommendation 1: Influenza viruses are prone

to developing drug-resistant strains. Monotherapy is likely to induce the selection of drug-resistant strains.Particular attention should be paid to influenza virus resistance in patients with severe influenza, children,and immunocompromised patients(Evidence Level:4,Recommendation Strength: C).

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4.2. The influence of drug resistance

4.2.1. Enhancement of virus spread ability

Some drug-resistant mutations of influenza virus may enhance the virus's transmission capacity. This enables the virus to spread rapidly in populations,especially among immunosuppressed patients. Seibert CW et al.(18) conducted a study using a guinea pig transmission model. They found that influenza viruses carrying S247N and H275Y mutations had high resistance to oseltamivir. These viruses also showed enhanced effective transmission ability. Hickerson et al.(19) noted that influenza viruses may develop drug-resistant mutations against baloxavir. These mutations include I38L and I38T. Additionally, IAV has emerged with the E199D mutation, and IBV has emerged with the I38T mutation. These initial mutations slightly impaired the virus's replication capacity. However,during continuous viral passage, IAV acquired the compensatory mutation D394N, while IBV evolved the E329G mutation. These subsequent mutations can enhance replication capacity of drug-resistant viruses.They also promote fixation of antiviral resistance in viral populations. Moreover, they facilitate further spread of such resistance. This poses a potential public health threat.

4.2.2. Increased risk of drug resistance gene spread

Influenza viruses(e.g., avian Influenza H5N1) may infect humans via cross-species transmission if they accumulate drug-resistant mutations in animal hosts.From 2003 to 2024, the WHO recorded 954 confirmed cases of human infection with highly pathogenic avian Influenza A(H5N1) virus across 24 countries.These cases resulted in 464 deaths, corresponding to a mortality rate of 48.64%(20). The highly pathogenic avian Influenza A(H5N1) virus of the 2.3.4.4b evolutionary branch was isolated from severe human cases in Chile. This virus showed high-titer replication ability in the respiratory and extrapulmonary tissues of ferrets(21). After the emergence of the self-evolved 2.3.4.4b branch, the highly pathogenic avian Influenza A(H5N1) virus has become a new recombinant virus with stronger cross-species transmission ability, capable of spreading widely among multiple mammalian species, including dairy cows, cats, and raccoons(22).The H5N1 virus of the 2.3.4.4b evolutionary branch has developed a new mutation through reassortment events.It possesses dual-receptor binding ability, enabling it to bind to both avian and human receptors. Through molecular adaptation, the H5N1 virus has enhanced its cross-species spread ability. This has led to its transmission in cattle, humans, and other mammals. It is recommended to adopt a multi-target antiviral regimen to reduce the risk of drug resistance(23).

4.2.3. Increased risk of clinical treatment failure and death

With development of drug resistance in influenza viruses,amantane drugs(such as amantadine and rimantadine)have become ineffective against most prevalent strains.Neuraminidase inhibitors(such as oseltamivir and zanamivir) were once the main treatment options.However, under drug selection pressure, influenza strains have also developed resistance to these drugs.Influenza drug-resistant strains can reduce the efficacy of monotherapy for influenza and may even lead to the failure of treatment(24).

4.2.4. Increased burden of public health

Patients infected with drug-resistant strains require longer hospital stays and more intensive supportive care(such as mechanical ventilation and Extracorporeal Membrane Oxygenation(ECMO)), which increases the pressure on the healthcare system. Patients with drug-resistant infections need to use high-cost alternative drugs(such as new polymerase inhibitors or combination therapies). The per capita treatment cost can increase by 5 to 10 times, leading to a squeeze of medical resources.Drug resistance reduces the effectiveness of antiviral drugs such as neuraminidase inhibitors; Large-scale drug reserves prepared in the early stage may fail to play their expected role, resulting in the waste of drug reserves.

Recommendation 2: Drug resistance of influenza viruses may lead to increased viral transmission ability and a high risk of drug-resistant gene spread(Level of evidence: 4); it may also result in the failure of clinical antiviral treatment, elevate the mortality rate among patients with severe influenza, and increase the burden on public health(Evidence Level: 4, Recommendation Strength: C).

4.3. The theoretical basis of combined antiviral therapy

Influenza virus has high genetic variability, making it prone to resistance to single drugs. To prevent this,strategies of combination therapy or drug enhancement are optional(25). Antiviral combination therapy acts on different viral replication links and exerts synergistic effects, reducing single-drug pressure to boost efficacy and lower resistance risk. The WHO's 2024 guidelines(26) do not recommend routine combination therapy;single drugs(e.g., osetlamvir, baloxavir marboxil)remain preferred. However, for severe influenza patients(e.g.,requiring mechanical ventilation/ECMO) or immunodeficient patients(e.g.,post-hematopoietic stem cell transplantation), if viral load does not drop significantly 48 hours after monotherapy, combination therapy(e.g., osetlamvir+ baloxavir marboxil) may be considered, subject to individual evaluation. According to the Diagnosis and Treatment Protocol for Influenza

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(2020 Edition)(27) and(2025 Edition)(28), severe/critical cases may have extended treatment courses based on etiological results. Combining drugs with the same mechanism or increasing dosages is not recommended,but combining those with different mechanisms is not ruled out.

Recommendation 3: For patients with severe influenza or suspected drug-resistant strain infection,after individualized assessment, combined antiviral treatment with drugs of different mechanisms of action and different targets can be considered(Evidence Level:5, Recommendation Strength:D).

5. Combination antiviral therapy regimen

To date, compared with single-drug treatment,combination therapy with virus-targeted drugs and host-targeted drugs has achieved more positive clinical outcomes. These outcomes include reducing viral shedding, shortening the duration of influenza-related symptoms, and decreasing the selection of drug-resistant variants. Notably, the combination of a virus-targeted drug with anti-inflammatory/immunomodulatory agents has become one of the most promising treatment approaches. A brief introduction to the combination therapy regimens is provided below.

5.1. Sequential monotherapy

Early studies have identified the possibility of single-drug sequential therapy for immunocompromised patients(29). Five patients who received allogenelic hematopoietic stem cell transplantation still had symptoms and shed influenza viruses after one or more oseltamivir courses,and were then given sequential balavorx therapy. Among the three patients with wild-type influenza virus infection,two achieved viral clearance after balavorx treatment,while another developed a baloxavir-resistant polymerase variant(I38T). Subsequent studies further validated this approach(30): in severe Influenza A(H5N6) cases where oseltamivir was ineffective, balavormar cybaril rapidly reduced patients' viral load and cytokine levels. In recent years, clinicians observed that some immunocompromised patients or elderly patients with chronic comorbidities still had high influenza virus nucleic acid load 5 days after single oseltamivir treatment or single balavormar boxil treatment, with no obvious improvement in pneumonia.Sequential use of these two drugs promoted viral nucleic acid negativity and prevented disease progression.Additionally, sequential therapy is safe with no drug-drug interactions, though verification via multicenter large-cohort studies is still required. It should be noted that for sequential treatment with balavorvar marboxil, the single adult dose must not exceed 40 mg. After elderly patients(≥65 years old), adjust the dose based on renal function,and use with caution if creatinine clearance is< 30 mL/min.

Recommendation 4: For immunocompromised patients or elderly patients with chronic underlying diseases, if their condition shows no significant improvement after standard antiviral treatment and they remain persistently positive for Influenza virus nucleic acid, sequential antiviral therapy is recommended. The recommended regimens include oseltamivir followed by baloxavir marboxil, or baloxavir marboxil followed by oseltamivir(Evidence Level: 4, Recommendation Strength:C).

5.2. The combination of virus-targeted drugs with different mechanisms

5.2.1. The combination of different NAIs

In mouse models, for A(H3N2) and wild-type A(H1N1)pdm09 viruses, zanamivir monotherapy was more effective than oseltamivir monotherapy or the oseltamivir-zanamivir combination; however,for the oseltamivir-resistant A(H1N1)pdm09 H275Y virus variant, combination therapy was comparable to zanamivir monotherapy, and both were superior to oseltamivir monotherapy(31). In the hollow fiber infection model(HFIM) system, combined treatment with oseltamivir(75 mg Q12h, t1/2: 8 h) and zanamivir(600 mg Q12h, t1/2: 2.5 h) remained effective against viruses resistant to both agents(32). In randomized controlled trials of adult seasonal influenza(mainly H3N2), oseltamivir-zanamivir combination therapy was not more effective than oseltamivir monotherapy, nor was it significantly better than zanamivir monotherapy(33); retrospective studies on adult H7N9 infection also showed that oseltamivir-peramivir combination therapy was not superior to oseltamivir monotherapy(34). A case report(35) indicated that in critically ill Influenza A patients receiving invasive ventilation and ECMO support, the combined regimen of oral oseltamivir and inhaled zanamivir failed to prevent disease deterioration. Existing evidence shows that for wild-type influenza viruses, NAls combination therapy does not consistently outperform monotherapy.Ital potential value may be limited to specific scenarios,such as when NAI-resistant strain infection is confirmed or highly suspected(e.g., in areas with prevalent resistant strains) and no better alternatives(such as baloxavir) are available, and it can be used as a tentative strategy.

Recommendation 5: Routine use of NAls combination therapy is not recommended for treating seasonal influenza or avian influenza(such as H7N9) infections.Examples of such combination therapy include oseltamivir combined with zanamivir or peramivir(Evidence Level: 1b, Recommendation Strength: A).

5.2.2. The combination of NAIs and RNA polymerase acid protein inhibitors

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An in vitro study found that combining baloxavir with neuraminidase inhibitors(e.g., oseltamivir, lanaminivir)exerted a significant synergistic effect. This effect enhanced the inhibitory activity against influenza virus(36). In ferret model experiments, therapeutic effects of baloxavir and oseltamivir were tested separately and in combination. Results showed that compared with monotherapy, combination therapy significantly reduced the upper respiratory tract virus titer in ferrets. It also significantly lowered the rate of drug-resistant virus generation. In ferrets treated with oseltamivir, a new oseltamivir-resistant mutation(NA/H275Y)was observed. This phenomenon was not detected in the combination therapy group(37). Clinically,combination of baloxavir and oseltamivir has shown relatively favorable effects in treating two patients after hematopoietic stem cell transplantation. For one patient,flu symptoms were rapidly relieved after receiving this combination therapy, and the virus test result turned negative. The other patient also showed a good early response to the same treatment but experienced virus recurrence in the later stage(38). Subsequently,international reports indicated that a 10-day regimen of zanamivir combined with baloxavir could effectively control the persistent replication of influenza virus in patients after hematopoietic stem cell transplantation(39).

5.2.3. The combination of NAIs and RNA polymerase basic protein inhibitors

A prospective study on adult influenza(40) showed that combining favipiravir with oseltamivir accelerated clinical recovery in patients with severe influenza. This effect was more significant than that of oseltamivir monotherapy. This treatment strategy deserves further evaluation in randomized controlled trials. A randomized double-blind trial(41) compared the pharmacokinetics and efficacy of modiporiv combined with oseltamivir.The trial included elderly and non-elderly hospitalized patients. Results showed the combination therapy group was safe and effective: its viral load was significantly lower than the placebo group, and symptom relief time was shorter(72.45 hours vs 94.15 hours). The incidence of influenza-related complications was also lower(7.9% vs 15.6%). Finberg RW et al.(42) found that compared with the placebo group, the modiporiv-oseltamivir combination group had a significantly lower viral load titer over time. The symptom relief time of the combination group also tended to be shorter than that of the placebo group. The early Phase II study showed positive results. Two subsequent key Phase III clinical trials were conducted in inpatients and high-risk outpatients. These trials failed to reach the primary endpoint. The research and development of this drug has been terminated. It should no longer be considered for clinical treatment.

5.2.4. The combination of NAIs and Envelope glycoprotein hemagglutinin inhibitors

MEDI8852 is a novel monoclonal antibody. In mouse and ferret models, the combination of MEDI8852 and oseltamivir significantly enhances therapeutic efficacy when treatment is delayed. Additionally, combining MEDI8852 with oseltamivir shows notable effects in preventing and treating Influenza A virus(H5N1 and H7N9) infections(43). A randomized, double-blind, placebo-controlled clinical trial found that the combination of oseltamivir and MEDI8852 is similar to oseltamivir monotherapy in reducing viral shedding.The combination treatment does not induce viral drug resistance changes and demonstrates good safety(44). Yi et al.(45) developed a new antibody mixture named CT-P27. In mouse models of influenza virus infection, CT-P27 exhibits in vivo therapeutic efficacy and preventive potential. It also shows a synergistic effect when used in combination with oseltamivir. In immunodeficient nude mouse models, researchers evaluated the triple therapy of favipiravir combined with two monoclonal antibodies(targeting the HA stem and HA receptor-binding sites). They found that single-drug or dual-drug combinations could inhibit viral replication but not completely eliminate the virus.However, the triple combination therapy successfully cleared the virus, enabling nude mice to survive for 188 days without any recurrence signs. No drug-resistant mutations were detected in this study, and the virus's adaptability was not affected either. This triple combination therapy includes favipiravir, anti-HA stem monoclonal antibody and anti-HA receptor-binding site monoclonal antibody. It provides the possibility of eradicating influenza virus in immunodeficient hosts, thereby offering a new treatment strategy for patients with severe influenza or immunodeficiency(46). Gaisina I et al.(47) found that combining the small-molecule IAV entry inhibitor ING-1466 with oseltamivir or baloxavir marboxil can synergistically enhance therapeutic efficacy.

Recommendation 6: Oseltamivir combined with baloxavir can effectively control influenza virus replication(Evidence Level: 2c, Recommendation Strength:B); Oseltamivir combined with favipiravir is superior to monotherapy in reducing influenza virus load for influenza treatment(Evidence Level: 2a,Recommendation Strength:B); Oseltamivir combined with hemagglutinin inhibitors(e.g., MEDI8852) helps reduce viral shedding and enhance therapeutic efficacy(Evidence Level: 1b, Recommendation Strength: A).

5.3. The combination of host-targeted drugs and antiviral drugs

5.3.1. The combination of nonsteroidal anti-inflammatory drug and Oseltamivir phosphate

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In Phase III clinical trials, combining oseltamivir with celecoxib significantly reduced patient mortality compared with oseltamivir monotherapy(48). Similarly, in Phase IIB/III clinical trials, the combination of clarithromycin-naproxen and oseltamivir produced two key effects: it significantly reduced the 30-day and 90-day mortality of hospitalized patients infected with H3N2 influenza, and shortened the overall hospital stay(49). In addition, a study on hospitalized children with influenza showed results:children treated with the combination of clarithromycin,naproxen and oseltamivir had a shorter fever resolution time than those treated with oseltamivir alone. Their influenza virus titer also decreased significantly faster(50).These studies suggest that the treatment regimen of non-steroidal anti-inflammatory drugs combined with oseltamivir has greater potential for influenza treatment.

5.3.2. The combination of immunomodulatory drugs and Oseltamivir phosphate

Long JS et al.(51) evaluated the effect of osetamivir combined with human interferon $\lambda$ on the drug resistance barrier of pandemic H1N1 virus strains A/Netherlands/602/2009(H1N1pdm09) via an in vitro infection model. Results showed osetamivir monotherapy led to rapid viral drug resistance via a single neuraminidase gene mutation, while combining with interferonλ significantly delayed the emergence of drug-resistant variants. Some literature has explored new drug development strategies, including targeting viral polymerase complexes(e.g.,PB1, PB2, PA)and leveraging host factors such as combining NAIs,polymerase inhibitors and immunomodulators like interferonλ. However, these strategies still need more clinical data to verify their broad applicability and safety(52).

Allotern is an immunomodulatory drug with antiviral activity against multiple viruses, including influenza virus. According to studies, when Allotern is used in combination with zanamivir, it can inhibit the production of inflammatory mediators and the migration of inflammatory cells to lung tissue. This effect effectively alleviates progression of lung inflammation induced by H1N1 Influenza virus(53).

Nitazoxanide(NTZ) belongs to the class of thiazole antibiotics, and it enhances the host's antiviral resistance by regulating the host's immune response. In vitro experiments have demonstrated that compared with oseltamivir or nitazoxanide monotherapy, the combination of these two drugs shows greater efficacy in preventing infection and shortening duration of viral shedding.Moreover, in animal models, this combined regimen not only significantly boosts the antiviral effect of oseltamivir but also successfully blocks the virus from spreading to the lower respiratory tract(54).

5.3.3. The combination of host-targeted drugs and

baloxavir or Oseltamivir phosphate

The antiviral activity of the MEK inhibitor ATR-002 was evaluated in A549 cells. Both its monotherapy and combination with baloxavir marboxil against wild-type influenza strains and drug-resistant strains(with PA-138T mutation) were tested via virus titer reduction assay and co-analysis. Results showed that ATR-002 exerted significant inhibitory effects on both wild-type and PA-138T mutant strains. When used in combination with baloxavir marboxil, it exhibited a synergistic effect: combination therapy reduced viral load more effectively, especially when targeting drug-resistant strains, and its inhibitory effect was significantly better than that of either single drug used alone. The combination of ATR-002 and baloxavir marboxil provides a new therapeutic strategy for overcoming baloxavir marboxil resistance. It is also expected to open up new avenues for the treatment of drug-resistant influenza(55).

The combination of four MEK inhibitors(PD-0325901, AZD-6244, AZD-8330 and RDEA-119) with oseltamivir significantly enhanced oseltamivir's antiviral activity(56). This combination therapy demonstrates the potential of MEK inhibitors and deserves further verification through preclinical in vitro and in vivo experiments.

Combination treatment with CXCR2 antagonists and antiviral drugs can significantly reduce the incidence and mortality of toxic and sublethal influenza infections(57,58). Hanlon et al.(59) demonstrated that M85, a novel antiviral compound, effectively inhibits influenza virus entry in mouse influenza models. It exerts this effect by targeting the host kinase PIK3C2β. In addition,M85 shows a synergistic effect when combined with oseltamivir.

Recommendation 7: Combining host-targeted drugs with baloxavir marboxil and/or oseltamivir can exert a synergistic effect. It can rapidly reduce viral load and the incidence of drug-resistant influenza virus strains(Evidence Level: 2b, Recommendation Strength:B). The mentioned host-targeted drugs include non-steroidal anti-inflammatory drugs(e.g., celecoxib,naproxen), immunomodulatory drugs(e.g., interferonλ,nitazoxanide), and other host-targeted drugs(e.g., MEK inhibitors).

6. Population applicable for combined antiviral therapy

Combination antiviral therapy may be required for patients with resistance to current anti-influenza viral agents/poor efficacy of antiviral therapy, as well as patients with severe influenza, immunocompromised patients, and other critically ill high-risk patients who may require combination antiviral therapy due to persistent viral replication.

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6.1. Patients infected with ineffective/resistant strains of antiviral therapy

The poor efficacy of anti-influenza virus treatment is defined as follows: after standardized use of anti-influenza virus drugs, the patient's symptoms do not improve as expected(e.g., fever[≥38°C] lasting more than three days, and persistent or aggravated symptoms such as cough). Another sign is the continuous replication of the virus, which can be observed through positive nucleic acid testing indicating active viral replication.The poor efficacy of anti-Influenza virus treatment is often closely associated with the"drug resistance" of influenza viruses and infection with"drug-resistant strains". Drug resistance is defined as a functional state.In this state, influenza viruses lose or weaken their sensitivity to drugs through genetic mutations and other means under drug pressure. Drug-resistant strains, by contrast, refer to individual viruses or virus populations.They carry specific drug-resistant mutations and can stably exhibit this"drug-resistant state", serving as specific carriers of the drug-resistant state. According to the WHO definition, for IAV, a strain is determined to be drug-resistant if the concentration(IC50) required for a drug to inhibit 50% of viral replication is more than 100 times higher than the normal value. If the IC50 is 10 to 100 times higher than the normal value,it indicates reduced sensitivity, which may affect therapeutic efficacy(60). To accurately identify drug resistance, current methods for detecting influenza virus drug resistance mainly include phenotypic analysis and genotypic analysis. Phenotypic analysis includes plaque reduction experiments, chemiluminescence methods,and fluorescence methods. Genotypic analysis includes real-time fluorescence quantitative PCR, digital PCR and other techniques. Technologies such as gene chips,CRISPR detection, and next-generation sequencing are still in the research stage(61). In clinical practice, the more common types of drug-resistant strains mainly include oseltamivir-resistant strains and baloxavir-resistant strains. The existence of such drug-resistant strains often directly leads to reduced efficacy or failure of the corresponding drugs. In clinical settings, these factors are of paramount importance when modifying treatment plans.

6.1.1. Oseltamivir resistant strains

For oseltamivir-resistant strains, combining favipiravir can restore the sensitivity of resistant viruses to antiviral drugs(62). Favipiravir can effectively inhibit the activity of the PB1 subunit of influenza viruses.It has inhibitory activity against influenza strains resistant to neuraminidase inhibitors and amantadine drugs. Meanwhile, it almost does not inhibit human DNA synthesis and has good safety(63,64). In vitro experiments have shown that the PB2 inhibitor pimodivir

has a synergistic antiviral effect with oseltamivir(65). This drug is effective against IAV, including neuraminidase inhibitor-resistant and amantadine-resistant strains. However, it is ineffective against IBV(66). In terms of clinical research, results from the TOPAZ trial(42) indicated that when treating patients with acute, uncomplicated seasonal influenza A, pimodivir monotherapy could reduce viral load in a dose-dependent manner. The efficacy was more significant when pimodivir was used in combination with oseltamivir. Another set of clinical research data shows that in high-risk outpatients, the combined treatment of pimodivir and oseltamivir can also shorten the duration of influenza symptoms(67).

6.1.2. Baloxavir resistant strains

For baloxavir-resistant strains(with PA/I38T or PA/E23K mutation), combination therapy has shown potential to delay the occurrence of drug resistance.Koszalka P et al.(37) reported that in ferret models, the combination of baloxavir and oseltamivir could reduce the selection pressure on viruses with reduced drug sensitivity. This in turn lowers the risk of drug resistance.Park et al.(68) further verified the effect of baloxavir marboxil/oseltamivir monotherapy or combination therapy on the drug-resistant substitution of A(H1N1)pdm09 virus during continuous passage in mice. Deep sequencing analysis showed that the PA-I38X amino acid substitution variant emerged in 67%(n=4/6) of the mouse virus populations treated with baloxavir marboxil and monotherapy. The combination of baloxavir marboxil and oseltamivir could inhibit the production of this variant,providing a therapeutic strategy to reduce influenza virus drug resistance. Guo X et al.(36) evaluated the antiviral effect of combining baloxavir with neuraminidase inhibitors on wild-type influenza viruses and drug-resistant mutant influenza viruses. The results showed that this combination had a significant synergistic effect.Given the rapid emergence of baloxavir resistance, these results are believed to provide a useful reference for influenza combination therapy.

Recommendation 8: For influenza patients infected with oseltamivir-resistant strains or with poor antiviral response to oseltamivir, combination of favipiravir and pimodivir is recommended for influenza virus antiviral treatment(Evidence Level: 2b, Recommendation Strength: B); For influenza patients infected with baloxavir-resistant strains or with poor antiviral response to baloxavir, combination with oseltamivir is recommended for influenza virus antiviral treatment(Evidence Level: 3a, Recommendation Strength: B);It is also recommended to select an individualized combination treatment regimen of antiviral drugs and host-targeted drugs.

6.2. Severe/critical influenza patients

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According to the Diagnosis and Treatment Plan for Influenza(2025)(28), adult influenza is defined as severe if any of the following criteria are met: 1.The respiratory rate is≥30 breaths per minute; 2. Oxygen saturation is≤93% during resting room air inhalation; 3. The ratio of arterial partial pressure of oxygen to fractional inspired oxygen(PaO2/FiO2) is≤300 mmHg; 4. Lung imaging shows that lesions progress by more than 50% within 24-48 hours. Childhood influenza is defined as severe if any of the following criteria are met: 1. Persistent high fever lasting more than 3 days; 2. Shortness of breath(≥30-60 breaths per minute, depending on age); 3. Oxygen saturation is≤93%; 4. Presence of symptoms such as drowsiness or convulsions; 5. Severe dehydration; 6.Exacerbation of underlying diseases. On the basis of severe influenza, a case is considered critical if any life-threatening manifestation occurs, including respiratory failure requiring mechanical ventilation, septic shock requiring vasoactive drugs, and organ failure(e.g., acute kidney injury or acute necrotizing encephalopathy).

Combination therapy may shorten the course of illness in patients with severe influenza, but the supporting evidence is limited(69). A prospective study on adults found that the combination of favipiravir and oseltamivir promotes clinical recovery in patients with severe influenza more quickly than oseltamivir monotherapy(40). Fukao K et al.(70) compared the efficacy of baloxavir marboxil monotherapy,oseltamivir monotherapy, and the combination of the two in mouse models infected with influenza virus. In vitro experiments showed that baloxavir marboxil and neuraminidase inhibitors could synergistically inhibit viral replication. In animal experiments, combination therapy was superior to monotherapy in reducing viral titers, mortality, and inflammatory responses. A post hoc analysis was conducted on the FLAGSTONE study(71). A total of 143 patients with severe nfluenza were included in the efficacy analysis, including those with immunosuppression, diabetes, or chronic lung disease.Among them, 92 patients received baloxavir combined with neuraminidase inhibitors(dual antiviral group),and 51 patients received neuraminidase inhibitors alone(single antiviral group). Compared with neuraminidase inhibitor monotherapy, the combination of baloxavir and neuraminidase inhibitors showed a better effect in reducing mortality. In the future, multicenter prospective cohort studies and randomized controlled trials need to be conducted to clarify the efficacy and safety of combination therapy in various critically ill patient groups.

Recommendation 9: For patients with severe or critical influenza, antiviral treatment is recommended as follows: one option is oseltamivir combined with favipiravir(Evidence Level: 2b, Recommendation Strength: B). The other option is oseltamivir combined with baloxavir(Evidence Level: 1b, Recommendation Strength: A).

6.3. Immunosuppressed/compromised patients

For influenza patients with weakened immune systems,their immune systems cannot effectively clear the virus, so they often need long-term treatment with neuraminidase inhibitors. Such patients excrete the virus for a long time, which is highly likely to induce drug-resistant mutations-these mutations seriously affect an antiviral efficacy and prolong infections. Thus,there is an urgent clinical need for strategies to rapidly and strongly inhibit viral replication, and sequential or combined antiviral therapy is particularly important.Mhamdi Z et al.(72) studied immunosuppressed mice infected with H3N2 virus, which received 10-day monotherapy or combination therapy with oseltamivir,favipiravir, or baloxavir. Results showed oseltamivir and favipiravir monotherapy only delayed mortality(average death days: 21.4, 24 vs. 11.4 in the untreated group).The combination of oseltamivir and favipiravir increased the survival rate to 80% and reduced lung viral titer;the combination of oseltamivir and baloxavir provided complete protection(100% survival) and significantly lowered lung viral titer. Oseltamivir and baloxavir monotherapy induced E119V(NA) and I38T(PA)substitutions respectively, but no resistance mutations were detected in the combination group-indicating combination therapy reduces drug resistance.

Clinical research on combined anti-influenza virus therapy in immunosuppressed populations has also made certain progress. Two patients with severe influenza pneumonia on immunosuppressive drugs had no virus clearance after 18 and 5 days of oseltamivir monotherapy respectively. After adding zanamivir and amantadine to their regimens, their throat swabs and plasma samples turned PCR-negative in 3 and 4 days respectively(73). In conclusion, existing evidence shows that for influenza patients with weakened immune function,although combined antiviral therapy can prolong survival or reduce viral load to some extent, a more potent, multi-target combination(e.g., triple combination of polymerase inhibitors, neuraminidase inhibitors,and monoclonal antibodies) may be needed to achieve complete clearance and avoid drug resistance. Future priority should be given to prospective clinical studies to verify the effectiveness and safety of such strategies in severely immunosuppressed populations.

Recommendation 10: For patients with suppressed or weakened immune function, sequential or combined treatment with oseltamivir, baloxavir, favipiravir, etc.can be selected(Evidence Level: 2c, Recommendation Strength: B).

6.4. Elderly patients aged≥65 years

Elderly individuals, especially those over 65 years old, often have multiple underlying diseases. This makes them prone to worsening existing conditions

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after influenza infection and may even trigger severe complications, that threaten their lives. For the elderly,the harm of influenza goes far beyond respiratory symptoms. Within a few weeks after acute infection, it may also induce extrapulmonary complications such as myocardial infarction or stroke, further increasing the medical burden. Studies have shown that elderly patients over 75 years old stay in the hospital for about two more days on average due to flu-related complications than those aged 50 to 64(74). In a Phase 2b clinical study(41), O'Neil B et al. compared the antiviral effects of modipivor combined with osetamivir versus oseltamivir monotherapy in non-elderly adults(18-64 years old) and elderly patients(65-85 years old). The results showed that the combination group significantly shortened the time to relief of influenza symptoms:72.45 hours for elderly patients and 94.15 hours for non-elderly patients. Moreover, the incidence of influenza-related complications in the combination group was also significantly lower than that in the monotherapy group,at 7.9% and 15.6% respectively. However, this evidence only comes from a single exploratory study and has not been fully validated in large-scale elderly populations. In addition, there is a lack of systematic assessment on the safety of combination treatment for this population.

Recommendation 11: For elderly patients aged≥65 years, individualized combined antiviral treatment can be formulated based on the patient's immune status,underlying diseases, and condition severity. Relevant recommendations for severe or critical cases and patients with suppressed or weakened immune function can be referred to(Evidence Level: 4, Recommendation Strength: C).

6.5. Pregnant women

Pregnant women and those within two weeks after childbirth are at high risk of influenza. Influenza may cause a series of complications in pregnant women,such as increased risks of premature birth, miscarriage,cesarean section, maternal respiratory failure, and death. Therefore, during the influenza season, it is recommended that pregnant women and women within two weeks after childbirth undergo influenza testing.Once diagnosed or suspected of having influenza,antiviral treatment should be initiated as soon as possible(75). For pregnant women, the preferred antiviral drug is oseltamivir. Small cohort studies have shown that inhaled zanamivir is safe for both pregnant women and exposed infants. However, relevant research data on baloxavir and peramivir in pregnant women are still insufficient.Favipiravir is contraindicated for pregnant women due to its reproductive toxicity(74). Therefore, the safety of antiviral drugs for pregnant women with influenza still requires further research, and combination antiviral therapy is not recommended at present.

$$\text{ Recommendation 12: For pregnant women with}$$

influenza, their condition changes should be closely monitored. Routine combined antiviral treatment for influenza is not recommended.(Evidence Level: 2a,Recommendation Strength: B).

6.6. Children

According to Practical Guidelines for Influenza Vaccination and Antiviral Drug Use in Children(2024 Edition)(76), for children with severe influenza and those infected with drug-resistant mutant strains, if their condition does not improve and continues to deteriorate 48 hours after initiating antiviral drug treatment,combination therapy of neuraminidase inhibitors and baloxavir or sequential therapy can be considered.The dosage and administration method should refer to the single-drug treatment standards. Baloxavir is only used for children aged 5 years and above; it is not recommended for children under 5 years old due to insufficient safety data. In addition, the course of combination treatment should not exceed 7 days.

Recommendation 13: Routine combination of antiviral drugs for the treatment of childhood influenza is not recommended. However, for children over 5 years old with severe conditions and weakened immune function, if the viral nucleic acid level is≤30, sequential use or combination of baloxavir and oseltamivir is recommended(Evidence Level: 1a, Recommendation Strength: A).

7. Considerations for combined antiviral therapy for influenza

7.1. Drug interactions

In studies on drug interactions in influenza antiviral combination therapy, the in vitro and in vivo safety as well as synergistic effects of different drug combinations have been preliminarily verified. In vitro studies have shown that the synergistic index and antagonistic index of ZSP1273 combined with oseltamivir are 852.41 and-0.19 respectively. This indicates a strong synergistic effect between the two drugs(77). At present, research on drug interactions in special populations is still relatively limited. In the future, further targeted studies are needed to improve medication guidance for different populations.

7.2. Overlapping risk of drug resistance

MU et al.(62)demonstrated that the combination of向下 aselamivir-resistant variants, thereby accelerating the evolution of resistance mutations. For instance, this combination could lead to the occurrence of PA/I38T+ H274Y double mutant strains. Thus, this antiviral combination regimen is not recommended for all

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influenza patients. Instead, it should be customized for specific populations, and the use of combination therapy requires individual assessment before decision-making.This approach enables more effective management of therapeutic risks and optimization of treatment outcomes.

7.3. Adverse effects and safety

Combination therapy of naproxen with oseltamivir was associated with a low incidence of adverse events(AEs) in one study(49). Another study demonstrated no statistically significant differences in AEs(including vomiting, diarrhea, and abdominal pain) when compared with oseltamivir monotherapy(50). In subjects receiving combination therapy with the monoclonal antibody MEDI8852 and oseltamivir, the incidence of AEs was relatively elevated. Nevertheless, the majority of these events were classified as mild or moderate, with bronchitis identified as the most frequent adverse event(44).By contrast, the combination of ZSP1273 and oseltamivir exhibited a favorable safety and tolerability profile, and no clinically significant drug-drug interactions were detected(78). A randomized controlled trial(RCT) observed that co-administration of baloxavir with a neuraminidase inhibitor did not increase the risk of AEs(71).

These studies indicate that although antiviral combination regimens are linked to AEs, their overall safety profile remains acceptable. This finding provides a scientific basis for clinicians to select appropriate drug combinations according to patients' specific conditions in clinical practice. Future studies could further explore the safety and efficacy of these regimens across different disease stages and drug doses, with the aim of continuously optimizing treatment strategies.

7.4. Cost-effectiveness

Jiang Y et al. constructed a linked dynamic communication economic evaluation model to assess the cost-effectiveness of osetamivir combined with baloxavir marboxil in the context of an influenza pandemic in China. The study results showed that adding baloxavir marboxil to the treatment regimen could reduce the cumulative incidence of influenza infection from 49.49% to 43.26% and increase quality-adjusted life years(QALYs). Compared with osetamivir monotherapy, the intervention regimen combined with baloxavir marboxil achieved a net monetary benefit of 77.85 yuan per person, with a willingness-to-pay(WTP)threshold of 80,976 yuan per QALY(79).

From the perspective of Japanese healthcare costs,baloxavir(used for post-exposure prophylaxis) and ranimivir(used for treatment) showed high cost-effectiveness. This means combining these two drugs for influenza prevention and treatment can achieve better therapeutic effects at lower costs. When using

the EuroQol-5 Dimension-5 Level(EQ-5D-5L) to measure health-related quality of life, the combination of baloxavir and raminivir also significantly improved patients' quality of life(80).

Baloxavir and ranimivir are not only cost-effective in healthcare but also have significant value in social and public health dimensions. Thus, this drug combination deserves further promotion and application in future influenza pandemic preparedness and response strategies.

7.5. Monitor

7.5.1. Monitoring of antiviral efficacy

Based on pharmacokinetic(PK) and pharmacodynamic(PD) data, anti-influenza drugs typically reach a plasma concentration plateau after 5-6 half-lives. Specifically,oseltamivir has a half-life of 6-10 hours, while zanamivir has a half-life of 2.5-5.1 hours. The efficacy of anti-influenza drugs can be assessed 48 hours after initiating the recommended treatment(81). It is recommended to recheck influenza nucleic acid 5 days after treatment initiation. For combined antiviral therapy, influenza virus nucleic acid changes can also be monitored at the above time points. For certain patients, the interval for influenza nucleic acid monitoring can be shortened appropriately, including those with risk factors for severe influenza, influenza patients with immunosuppression or impaired immune function, patients with severe or critical influenza, and patients whose condition worsens during treatment.

7.5.2. Monitoring of antiviral efficacy

From global surveillance data, the resistance rate of influenza viruses to neuraminidase inhibitors(NAIs,e.g.,oseltamivir, peramiriv) showed a slight upward trend(0.09%-0.23%) globally from 2020 to 2023.The incidence of baloxavir susceptibility reduction events also remained relatively low(0.07%-0.12%)(82). Although the current overall resistance rate is low, vigilance is needed regarding the epidemiological changes of drug-resistant strains in long-term monitoring.These changes directly determine clinical treatment efficacy and the success of public health prevention and control strategies, so risks must be managed through systematic monitoring and targeted responses.

According to the China Influenza Surveillance Weekly Report, the Influenza A(H1N1)pdm09 strain in China has remained generally sensitive to oseltamivir since 2009.However, local transmission of the H275Y mutant strain has been detected in some regions, and the resistance of this subtype to NAls still requires close attention(83).In clinical practice, regular monitoring of drug-resistant mutations is essential, especially for severe patients with persistent viral replication. If the main cause of disease progression is confirmed to be persistent viral positivity

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leading to elevated inflammatory factors, sequential or combined antiviral therapy should be adopted.Alternatively, antiviral drugs can be combined with host-targeting monoclonal antibodies or immunomodulators.Meanwhile, the turnaround time for drug resistance testing should be optimized, and rapid PCR technology should be used to detect specific mutations, facilitating timely adjustment of treatment strategies. Additionally,when evaluating new drugs or formulating combination strategies for immunosuppressed or critically ill patients,emphasis should be placed on analyzing drug resistance.Medical institutions with appropriate testing capabilities are recommended to perform drug resistance gene testing before initiating combined antiviral therapy. They should also conduct such testing if the influenza virus nucleic acid CT value is<30 five days after the start of antiviral treatment(74).

Recommendation 14: The overall safety of combined anti-influenza virus therapy is favorable.However, attention should still be paid to adverse drug reactions and drug interactions(Evidence Level:2a, Recommendation Strength: B). During combined therapy, monitoring of influenza virus nucleic acid load is recommended. Medical institutions with testing capabilities may conduct drug resistance gene testing(Evidence Level: 5, Recommendation Strength: D).

8. Future research directions

The rapid evolution of influenza viruses and the spread of drug resistance have become major threats to global public health. To address this challenge, researchers are conducting in-depth explorations from multiple dimensions, including molecular mechanisms, model prediction, and immune mechanisms, with the aim of innovating existing prevention, control, and treatment strategies. Researchers identified novel allosteric drug-resistant mutations of neuraminidase via deep mutational scanning. This study revealed a new mechanism: these mutations do not directly act on the drug-binding pocket but cause resistance by affecting protein tetramerization(84). A site-dynamics-based evolutionary model and a machine learning method were developed in two studies,respectively. These tools can more accurately predict the antigenic evolution of influenza viruses, especially H3N2, and provide powerful computational support for vaccine strain selection(85,86). Yang B et al. found that the breadth of influenza antibody cross-reactivity is affected by viral subtypes and exposure history. Repeated exposure to H3N2 virus can shape and expand the scope of human antibody responses, which lays a framework for understanding immune imprinting and vaccine design(87).

Looking ahead, future research directions mainly include the following: developing novel broad-spectrum antiviral drugs and new influenza vaccines; establishing a national influenza virus drug resistance monitoring

network that integrates real-time sequencing data and develops an artificial intelligence(AI)-based model for predicting viral evolution and drug resistance to provide early warning of drug-resistant mutations; developing individualized precision treatment strategies based on machine learning and AI algorithms to automatically recommend optimal single-drug or combination therapy regimens; and conducting multicenter RCTs to evaluate the safety and long-term efficacy of combination therapy in preventing severe cases and in populations such as immunocompromised patients and children.

Acknowledgements

We thank all the participants in this study.

Funding: This work was supported by grants from the Shenzhen Clinical Research Center for Emerging Infectious Diseases(No. LCYSSQ20220823091203007),Shenzhen Medical Research Fund(B2402042), and Shenzhen High-level Hospital Construction Fund.

Conflict of Interest: The authors have no conflicts of interest to disclose.

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Received October 4, 2025; Revised October 25, 2025; Accepted October 27,2025.

These authors contributed equally to this work.

*Address correspondence to:

Hongzhou Lu, National Clinical Research Center for Infectious Diseases, Shenzhen Third People's Hospital, The Second Affiliated Hospital Southern University of Science and Technology, Shenzhen, Guangdong, China;

E-mail:luhongzhou@szys.sustech.edu.cn

Guangbin Chen, Department of Pharmacy, The Third People's Hospital of Shenzhen, Shenzhen, Guangdong, China;E-mail: pts666666@126.com

Released online in J-STAGE as advance publication October 29,2025.