The potential impact of neuraminidase inhibitor resistant influenza

The potential for multidrug-resistant influenza

PURPOSE OF REVIEW

The 2009 influenza pandemic introduced a new influenza A/H1N1 subtype in the human population. This pandemic 2009 influenza A/H1N1 virus has natural resistance to the adamantanes class and has a low threshold to become resistant to the neuraminidase class of antiviral drugs. This review describes recent findings on influenza antiviral resistance in pandemic 2009 influenza A/H1N1 virus.

RECENT FINDINGS

Pandemic 2009 viruses have emerged with novel resistance patterns to the neuraminidase inhibitors. In addition, the identification of mutations that facilitated oseltamivir resistance in prepandemic influenza emphasizes the ability of influenza to become resistant to antiviral drugs without significant loss of fitness.

SUMMARY

Novel initiatives are required to find and develop high genetic barrier influenza therapeutic regimens for effective treatment of severe influenza virus infections.

Cost-effectiveness of oseltamivir treatment for children with uncomplicated seasonal influenza

OBJECTIVE

To evaluate the cost-effectiveness of oseltamivir treatment for seasonal influenza in children and consider the impact of oseltamivir resistance on these findings.

STUDY DESIGN

We developed a model to evaluate 1-year clinical and economic outcomes associated with 3 outpatient management strategies for unvaccinated children with influenza-like-illness: no antiviral treatment; diagnostic testing and oseltamivir treatment when positive; and empiric oseltamivir treatment. The model depicted a hypothetical non-pandemic influenza season with a 29% level of oseltamivir resistance in circulating viruses, and 14% to 54% probability of seasonal influenza with influenza-like-illness. Strategies were compared with incremental cost-effectiveness ratios.

RESULTS

In our primary analysis, empiric oseltamivir treatment consistently produced the greatest benefit. The incremental cost-effectiveness of this alternative, compared with testing and treating, was <$100,000 per quality-adjusted life year gained in all age groups except the oldest. The testing strategy was consistently more effective compared with no treatment and cost between $25,900 and $71,200 per quality-adjusted life year gained, depending on age. Results were sensitive to the prevalence of oseltamivir resistance in circulating viruses.

CONCLUSION

Empiric oseltamivir treatment of seasonal influenza is associated with favorable cost-effectiveness ratios, particularly in children aged 1 to <12 years, but ratios are highly dependent on the prevalence of oseltamivir resistance among circulating influenza viruses.

Disruption of virus-host cell interactions and cell signaling pathways as an anti-viral approach against influenza virus infections

Influenza is still one of the major plagues worldwide with the threatening potential to cause pandemics. In recent years, increasing levels of resistance to the four FDA approved anti-influenza virus drugs have been described. This situation underlines the urgent need for novel anti-virals in preparation for future influenza epidemics or pandemics. Although the anti-virals currently in use target viral factors such as the neuraminidase or the M2 ion channel, there is an increase in pre-clinical approaches that focus on cellular factors or pathways that directly or indirectly interact with virus replication. This does not only include inhibitors of virus-supportive signaling cascades but also interaction blockers of viral proteins with host cell proteins. This review aims to highlight some of these novel approaches that represent a paradigm change in anti-viral strategies against the influenza virus. Although most of these approaches are still in an early phase of preclinical development they might be very promising particularly with respect to the prevention of viral resistance to potential drugs.

Inhibition of influenza virus replication by constrained peptides targeting nucleoprotein

BACKGROUND

Because of high mutation rates, new drug-resistant viruses are rapidly evolving, thus making the necessary control of influenza virus infection difficult.

METHODS

We screened a constrained cysteine-rich peptide library mimicking μ-conotoxins from Conus geographus and a proline-rich peptide library mimicking lebocin 1 and 2 from Bombyx mori by using influenza virus RNA polymerase (PB1, PB2 and PA) and nucleoprotein (NP) as baits.

RESULTS

Among the 22 peptides selected from the libraries, we found that the NP-binding proline-rich peptide, PPWCCCSPMKRASPPPAQSDLPATPKCPP, inhibited influenza replicon activity to mean±sd 40.7%±15.8% when expressed as a GFP fusion peptide in replicon cells. Moreover, when the GFP fusion peptide was transduced into cells by an HIV-TAT protein transduction domain sequence, the replication of influenza virus A/WSN/33 (WSN) at a multiplicity of infection of 0.01 was inhibited to 20% and 69% at 12 and 24 h post-infection, respectively. In addition, the TAT-GFP fusion peptide was able to slightly protect Balb/c mice from WSN infection when administrated prior to the infection.

CONCLUSIONS

These results suggest the potential of this peptide as the seed of an anti-influenza drug and reveal the usefulness of the constrained peptide strategy for generating inhibitors of influenza infection. The results also suggest that influenza NP, which is conserved among the influenza A viruses, is a good target for influenza inhibition, despite being the most abundant protein in infected cells.

Pandemic influenza and health system resource gaps in Bali: an analysis through a resource transmission dynamics model

The failure to contain pandemic influenza A(H1N1) 2009 in Mexico has shifted global attention from containment to mitigation. Limited surveillance and reporting have, however, prevented detailed assessment of mitigation during the pandemic, particularly in low- and middle-income countries. To assess pandemic influenza case management capabilities in a resource-limited setting, the authors used a health system questionnaire and density-dependent, deterministic transmission model for Bali, Indonesia, determining resource gaps. The majority of health resources were focused in and around the provincial capital, Denpasar; however, gaps are found in every district for nursing staff, surgical masks, and N95 masks. A relatively low pathogenicity pandemic influenza virus would see an overall surplus for physicians, antivirals, and antimicrobials; however, a more pathogenic virus would lead to gaps in every resource except antimicrobials. Resources could be allocated more evenly across Bali. These, however, are in short supply universally and therefore redistribution would not fill resource gaps.

XIth International Symposium on Respiratory Viral Infections

Please cite this paper as: Belser et al. (2011) XIth International Symposium on Respiratory Viral Infections. Influenza and Other Respiratory Viruses 5(6), 443–e457.

Clinical and socioeconomic impact of different types and subtypes of seasonal influenza viruses in children during influenza seasons 2007/2008 and 2008/2009

Background

There are few and debated data regarding possible differences in the clinical presentations of influenza A/H1N1, A/H3N2 and B viruses in children. This study evaluates the clinical presentation and socio-economic impact of laboratory-confirmed influenza A/H1N1, A/H3N2 or B infection in children attending an Emergency Room because of influenza-like illness.

Methods

Among the 4,726 children involved, 662 had influenza A (143 A/H1N1 and 519 A/H3N2) and 239 influenza B infection detected by means of real-time polymerase chain reaction. Upon enrolment, systematic recordings were made of the patients' demographic characteristics and medical history using standardised written questionnaires. The medical history of the children was re-evaluated 5-7 days after enrolment and until the resolution of their illness by means of interviews and a clinical examination by trained investigators using standardised questionnaires. During this evaluation, information was also obtained regarding illnesses and related morbidity among households.

Results

Children infected with influenza A/H1N1 were significantly younger (mean age, 2.3 yrs) than children infected with influenza A/H3N2 (mean age, 4.7 yrs; p < 0.05)) or with influenza B (mean age, 5.2 yrs; p < 0.05). Adjusted for age and sex, children with influenza A/H3N2 in comparison with those infected by either A/H1N1 or with B influenza virus were more frequently affected by fever (p < 0.05) and lower respiratory tract involvement (p < 0.05), showed a worse clinical outcome (p < 0.05), required greater drug use (p < 0.05), and suffered a worse socio-economic impact (p < 0.05). Adjusted for age and sex, children with influenza B in comparison with those infected by A/H1N1 influenza virus had significantly higher hospitalization rates (p < 0.05), the households with a disease similar to that of the infected child (p < 0.05) and the need for additional household medical visits (p < 0.05).

Conclusions

Disease due to influenza A/H3N2 viral subtype is significantly more severe than that due to influenza A/H1N1 subtype and influenza B virus, which indicates that the characteristics of the different viral types and subtypes should be adequately considered by health authorities when planning preventive and therapeutic measures.

Rapid discrimination of oseltamivir-resistant 275Y and -susceptible 275H substitutions in the neuraminidase gene of pandemic influenza A/H1N1 2009 virus by duplex one-step RT-PCR assay

Pandemic influenza A/H1N1 2009 (A/H1N1pdm) virus caused significant outbreaks worldwide last year (2009). A number of oseltamivir‐resistant A/H1N1pdm viruses possessing an H275Y substitution in the neuraminidase (NA) protein were reported sporadically in several countries, including Japan, but they were sensitive to zanamivir and did not spread in the community. In this study, to monitor rapidly and simply oseltamivir‐resistant A/H1N1pdm viruses possessing H275Y, a duplex one‐step RT‐PCR assay (H275Y RT‐PCR assay) was developed based on an endpoint genotyping analysis method. H275Y RT‐PCR assay evaluated using several subtypes/types of influenza A and B viruses and other respiratory pathogenic viruses and shown to have high sensitivity and high specificity. Forty‐four clinical specimens were tested after RNA purification using the H275Y RT‐PCR assay, resulting in one clinical specimen being found to contain a virus possessing the H275Y mutation. Seventy‐three clinical isolates were then tested with the H275Y assay by using clinical isolates in the cultured supernatants of cells directly, without RNA purification, and the results were consistent with the NA sequencing. Since the H275Y RT‐PCR assay could detect the H275Y mutation in clinical isolates without RNA purification, as well as a H275Y mutated virus in clinical specimens after RNA purification, the assay was considered a powerful tool for surveillance screening of oseltamivir‐resistant A/H1N1pdm virus activity. J. Med. Virol. 83:1121–1127, 2011. © 2011 Wiley‐Liss, Inc.

High-resolution melting approach to efficient identification and quantification of H275Y mutant influenza H1N1/2009 virus in mixed-virus-population samples

The single-nucleotide variation 823C to T (His275Tyr), responsible for oseltamivir drug resistance has been detected in some isolates of the influenza A/H1N1/2009 virus. Early detection of the presence of this oseltamivir-resistant strain allows prompt consideration of alternative treatment options. An isolated-probe-asymmetric amplification PCR (Roche LightCycler v2.0) and high-resolution melting (HRM) method using unlabeled probes and amplified products (Idaho LightScanner 32) was designed and optimized to detect and estimate the proportion of H275Y mutants in influenza A/H1N1/2009 virus samples. The lower limit of quantification within the linear range of PCR assay detection was 200 copies/reaction. The melting peaks of the H275Y-specific unlabeled probe for the wild-type A/H1N1/2009 and H275Y mutant viruses were clearly distinguishable at 65.5°C and 69.0°C, respectively, at various ratios of wild-type/mutant virus population standards. The 95% detection limit for the 10% mutant sample pool was 1,200 copies/reaction (95% confidence interval, 669.7 to 3,032.6 copies/reaction). This HRM assay was tested with 116 archived clinical specimens. The quantitative HRM results obtained with samples containing mixed mutant-wild-type virus populations, at threshold cycle (C(T)) values of <29, compared well to those obtained with a pyrosequencing method performed by an independent laboratory. The quantitative feature of this assay allows the proportions of mutant and wild-type viral populations to be determined, which may assist in the conventional clinical management of infected patients and potentially more preemptive clinical management. This validated quantitative HRM method, with its low running cost, is well positioned as a rapid, high-throughput screening tool for oseltamivir resistance mutations in influenza A/H1N1/2009 virus-infected patients, with the potential to be adapted to other influenza virus species.

H275Y mutant pandemic (H1N1) 2009 virus in immunocompromised patients

Most oseltamivir-resistant pandemic (H1N1) 2009 viruses have been isolated from immunocompromised patients. To describe the clinical features, treatment, outcomes, and virologic data associated with infection from pandemic (H1N1) 2009 virus with H275Y mutation in immunocompromised patients, we retrospectively identified 49 hematology-oncology patients infected with pandemic (H1N1) 2009 virus. Samples from 33 of those patients were tested for H275Y genotype by allele-specific real-time PCR. Of the 8 patients in whom H275Y mutations was identified, 1 had severe pneumonia; 3 had mild pneumonia with prolonged virus shedding; and 4 had upper respiratory tract infection, of whom 3 had prolonged virus shedding. All patients had received oseltamivir before the H275Y mutation was detected; 1 had received antiviral prophylaxis. Three patients excreted resistant virus for >60 days. Emergence of oseltamivir resistance is frequent in immunocompromised patients infected with pandemic (H1N1) 2009 virus and can be associated with a wide range of clinical disease and viral kinetics.

Strategies for the use of oseltamivir and zanamivir during pandemic outbreaks

BACKGROUND

The use of neuraminidase inhibitors (oseltamivir and zanamivir) for the treatment of ill individuals has been an important intervention during the 2009 H1N1 pandemic. However, the emergence and spread of drug resistance remains a major concern and, therefore, optimizing antiviral strategies is crucial to retain the long-term effectiveness of these pharmaceutical interventions.

METHODS

A dynamic model of disease transmission was developed to investigate optimal scenarios for the use of a secondary drug (eg, zanamivir). Considering both small and large stockpiles, attack rates were projected by simulating the model to identify 'tipping points' for switching to zanamivir as resistance to oseltamivir develops.

RESULTS

The use of a limited stockpile of zanamivir can substantially reduce the overall attack rate during pandemic outbreaks. For a reasonably large stockpile of zanamivir, it is optimal to delay the use of this drug for a certain amount of time during which oseltamivir is used as the primary drug. For smaller stockpiles, however, earlier use of zanamivir will be most effective in reducing the overall attack rate. Given a limited stockpile of zanamivir (1.8% in the Canadian plan) without replenishment, and assuming that the fraction of ill individuals being treated is maintained below 60%, the results suggest that zanamivir should be dispensed as the primary drug for thresholds of the cumulative number of oseltamivir resistance below 20%.

INTERPRETATION

Strategic use of a secondary drug becomes crucial for pandemic mitigation if vaccination and other interventions fail to sufficiently reduce disease transmission in the community. These findings highlight the importance of enhanced surveillance and clinical monitoring for rapid identification of resistance emergence and its population incidence, so that optimal timing for adaptation to the use of drugs can be achieved.

Oseltamivir-resistant pandemic A(H1N1) 2009 influenza viruses detected through enhanced surveillance in the Netherlands, 2009-2010

Enhanced surveillance of infections due to the pandemic A(H1N1) influenza virus, which included monitoring for antiviral resistance, was carried out in the Netherlands from late April 2009 through late May 2010. More than 1100 instances of infection with the pandemic A(H1N1) influenza virus from 2009 and 2010 [A(H1N1) 2009] distributed across this period were analyzed. Of these, 19 cases of oseltamivir-resistant virus harboring the H275Y mutation in the neuraminidase (NA) were detected. The mean 50% inhibitory concentration (IC50) levels for oseltamivir- and zanamivir-susceptible A(H1N1) 2009 viruses were 1.4-fold and 2-fold, respectively, lower than for the seasonal A(H1N1) influenza viruses from 2007/2008; for oseltamivir-resistant A(H1N1) 2009 virus the IC50 was 2.9-fold lower. Eighteen of the 19 patients with oseltamivir-resistant virus showed prolonged shedding of the virus and developed resistance while on oseltamivir therapy. Sixteen of these 18 patients had an immunodeficiency, of whom 11 had a hematologic disorder. The two other patients had another underlying disease. Six of the patients who had an underlying disease died; of these, five had received cytostatic or immunosuppressive therapy. No indications for onward transmission of resistant viruses were found. This study showed that the main association for the emergence of cases of oseltamivir-resistant A(H1N1) 2009 virus was receiving antiviral therapy and having drug-induced immunosuppression or an hematologic disorder. Except for a single case of a resistant virus not linked to oseltamivir therapy, the absence of detection of resistant variants in community specimens and in specimens from contacts of cases with resistant virus suggested that the spread of resistant A(H1N1) 2009 virus was limited. Containment may have been the cumulative result of impaired NA function, successful isolation of the patients, and prophylactic measures to limit exposure.

Novel genotyping and quantitative analysis of neuraminidase inhibitor resistance-associated mutations in influenza a viruses by single-nucleotide polymorphism analysis

Neuraminidase (NA) inhibitors are among the first line of defense against influenza virus infection. With the increased worldwide use of the drugs, antiviral susceptibility surveillance is increasingly important for effective clinical management and for public health epidemiology. Effective monitoring requires effective resistance detection methods. We have developed and validated a novel genotyping method for rapid detection of established NA inhibitor resistance markers in influenza viruses by single nucleotide polymorphism (SNP) analysis. The multi- or monoplex SNP analysis based on single nucleotide extension assays was developed to detect NA mutations H275Y and I223R/V in pandemic H1N1 viruses, H275Y in seasonal H1N1 viruses, E119V and R292K in seasonal H3N2 viruses, and H275Y and N295S in H5N1 viruses. The SNP analysis demonstrated high sensitivity for low-content NA amplicons (0.1 to 1 ng/μl) and showed 100% accordant results against a panel of defined clinical isolates. The monoplex assays for the H275Y NA mutation allowed precise and accurate quantification of the proportions of wild-type and mutant genotypes in virus mixtures (5% to 10% discrimination), with results comparable to those of pyrosequencing. The SNP analysis revealed the lower growth fitness of an H275Y mutant compared to the wild-type pandemic H1N1 virus by quantitatively genotyping progeny viruses grown in normal human bronchial epithelial cells. This novel method offers high-throughput screening capacity, relatively low costs, and the wide availability of the necessary equipment, and thus it could provide a much-needed approach for genotypic screening of NA inhibitor resistance in influenza viruses.

The aminobisphosphonate pamidronate controls influenza pathogenesis by expanding a gammadelta T cell population in humanized mice

As shown in humanized mice, a population of Vγ9Vδ2 T cells can reduce the severity and mortality of disease caused by infection with human and avian influenza viruses.

There are few antiviral drugs for treating influenza, and the emergence of antiviral resistance has further limited the available therapeutic options. Furthermore, antivirals are not invariably effective in severe influenza, such as that caused by H5N1 viruses. Thus, there is an urgent need to develop alternative therapeutic strategies. Here, we show that human Vγ9Vδ2 T cells expanded by the aminobisphosphonate pamidronate (PAM) kill influenza virus–infected cells and inhibit viral replication in vitro. In Rag2^−/−γc^−/− immunodeficient mice reconstituted with human peripheral mononuclear cells (huPBMCs), PAM reduces disease severity and mortality caused by human seasonal H1N1 and avian H5N1 influenza virus, and controls the lung inflammation and viral replication. PAM has no such effects in influenza virus–infected Rag2^−/−γc^−/− mice reconstituted with Vγ9Vδ2 T cell–depleted huPBMCs. Our study provides proof-of-concept of a novel therapeutic strategy for treating influenza by targeting the host rather than the virus, thereby reducing the opportunity for the emergence of drug-resistant viruses. As PAM has been commonly used to treat osteoporosis and Paget’s disease, this new application of an old drug potentially offers a safe and readily available option for treating influenza.

Oseltamivir-resistant influenza A and B viruses pre- and postantiviral therapy in children and young adults with cancer

BACKGROUND

Immunocompromised patients are highly susceptible to influenza infection and can have prolonged viral shedding, which is a risk factor for the development of antiviral resistance.

METHODS

We investigated the emergence of oseltamivir-resistant influenza variants in children and young adults with cancer during the 2002-2008 influenza seasons. The demographic and clinical features of influenza infections in 12 patients who had viral isolates obtained before and after oseltamivir therapy was initiated were studied. Antiviral susceptibilities were determined by the fluorescence-based neuraminidase (NA) enzyme inhibition assay and by sequencing genes encoding NA and matrix M2 proteins.

RESULTS

The mean age of patients was 10.5 (range, 1.1-23.0) years. Ten patients had hematologic malignancies, 4 were recipients of hematopoietic stem cell transplants, and all patients were receiving immunosuppressive therapy. Eleven patients had prolonged respiratory symptoms and 8 had prolonged viral shedding. Serial viral isolates were available for 8 of 12 patients. Oseltamivir-resistant influenza viruses were isolated from 4 children (3 influenza A [H3N2] and 1 influenza B virus): before the initiation of antiviral therapy in 2 patients and during therapy in the other 2 patients. Three resistant influenza A (H3N2) viruses shared a common E119V NA mutation. One patient was infected with oseltamivir-resistant influenza B virus (IC50, 731.86 ± 155.12 nM) that harbored a N294S NA mutation, the first report of this mutation in influenza B viruses.

CONCLUSIONS

Oseltamivir-resistant influenza viruses can exist before or rapidly emerge during antiviral therapy in immunocompromised individuals, and this has important implications for therapy and infection control.

Assessing the in vitro fitness of an oseltamivir-resistant seasonal A/H1N1 influenza strain using a mathematical model

In 2007, the A/Brisbane/59/2007 (H1N1) seasonal influenza virus strain acquired the oseltamivir-resistance mutation H275Y in its neuraminidase (NA) gene. Although previous studies had demonstrated that this mutation impaired the replication capacity of the influenza virus in vitro and in vivo, the A/Brisbane/59/2007 H275Y oseltamivir-resistant mutant completely out-competed the wild-type (WT) strain and was, in the 2008–2009 influenza season, the primary A/H1N1 circulating strain. Using a combination of plaque and viral yield assays, and a simple mathematical model, approximate values were extracted for two basic viral kinetics parameters of the in vitro infection. In the ST6GalI-MDCK cell line, the latent infection period (i.e., the time for a newly infected cell to start releasing virions) was found to be 1–3 h for the WT strain and more than 7 h for the H275Y mutant. The infecting time (i.e., the time for a single infectious cell to cause the infection of another one) was between 30 and 80 min for the WT, and less than 5 min for the H275Y mutant. Single-cycle viral yield experiments have provided qualitative confirmation of these findings. These results, though preliminary, suggest that the increased fitness success of the A/Brisbane/59/2007 H275Y mutant may be due to increased infectivity compensating for an impaired or delayed viral release, and are consistent with recent evidence for the mechanistic origins of fitness reduction and recovery in NA expression. The method applied here can reconcile seemingly contradictory results from the plaque and yield assays as two complementary views of replication kinetics, with both required to fully capture a strain's fitness.

Neuraminidase inhibitor sensitivity and receptor-binding specificity of Cambodian clade 1 highly pathogenic H5N1 influenza virus

The evolution of the highly pathogenic H5N1 influenza virus produces genetic variations that can lead to changes in antiviral susceptibility and in receptor-binding specificity. In countries where the highly pathogenic H5N1 virus is endemic or causes regular epidemics, the surveillance of these changes is important for assessing the pandemic risk. In Cambodia between 2004 and 2010, there have been 26 outbreaks of highly pathogenic H5N1 influenza virus in poultry and 10 reported human cases, 8 of which were fatal. We have observed naturally occurring mutations in hemagglutinin (HA) and neuraminidase (NA) of Cambodian H5N1 viruses that were predicted to alter sensitivity to neuraminidase inhibitors (NAIs) and/or receptor-binding specificity. We tested H5N1 viruses isolated from poultry and humans between 2004 and 2010 for sensitivity to the NAIs oseltamivir (Tamiflu) and zanamivir (Relenza). All viruses were sensitive to both inhibitors; however, we identified a virus with a mildly decreased sensitivity to zanamivir and have predicted that a V149A mutation is responsible. We also identified a virus with a hemagglutinin A134V mutation, present in a subpopulation amplified directly from a human sample. Using reverse genetics, we verified that this mutation is adaptative for human α2,6-linked sialidase receptors. The importance of an ongoing surveillance of H5N1 antigenic variance and genetic drift that may alter receptor binding and sensitivities of H5N1 viruses to NAIs cannot be underestimated while avian influenza remains a pandemic threat.

Frequency of drug-resistant viruses and virus shedding in pediatric influenza patients treated with neuraminidase inhibitors

BACKGROUND

Although influenza virus resistance to the neuraminidase inhibitor zanamivir is reported less frequently than is resistance to the neuraminidase inhibitor oseltamivir in clinical settings, it is unknown whether this difference is due to the limited use of zanamivir or to an inherent property of the drug. We therefore compared the prevalence of drug-resistant viruses and virus shedding in seasonal influenza virus-infected children treated with either oseltamivir or zanamivir.

METHODS

Clinical specimens (throat or nasal swab) were collected from a total of 144 pediatric influenza patients during the 2005-2006, 2006-2007, 2007-2008, and 2008-2009 influenza seasons. Neuraminidase inhibitor-resistant mutants were detected among the isolated viruses by sequencing the viral hemagglutinin and neuraminidase genes. Sensitivity of the viruses to neuraminidase inhibitors was tested by neuraminidase inhibition assay.

RESULTS

In oseltamivir- or zanamivir-treated influenza patients who were statistically comparable in their age distribution, vaccination history, and type or subtype of virus isolates, the virus-shedding period in zanamivir-treated patients was significantly shorter than that in oseltamivir-treated patients. Furthermore, the frequency of zanamivir-resistant viruses was significantly lower than that of oseltamivir-resistant viruses.

CONCLUSION

In comparison with treatment with oseltamivir, treatment of pediatric patients with zanamivir resulted in the emergence of fewer drug-resistant influenza viruses and a shorter virus-shedding period. We conclude that zanamivir shows promise as a better therapy for pediatric influenza patients.

Dual resistance to adamantanes and oseltamivir among seasonal influenza A(H1N1) viruses: 2008-2010

Two distinct genetic clades of seasonal influenza A(H1N1) viruses have cocirculated in the recent seasons: clade 2B oseltamivir-resistant and adamantane-susceptible viruses, and clade 2C viruses that are resistant to adamantanes and susceptible to oseltamivir. We tested seasonal influenza A(H1N1) viruses collected in 2008-2010 from the United States and globally for resistance to antivirals approved by the Food and Drug Administration. We report 28 viruses with both adamantane and oseltamivir (dual) resistance from 5 countries belonging to 4 distinct genotypes. Because of limited options for antiviral treatment, emergence of dual-resistant influenza viruses poses a public health concern, and their circulation needs to be closely monitored.

Early emergence of an H275Y mutation in a hematopoietic cell transplant recipient treated with intravenous peramivir

Oseltamivir resistance in pandemic 2009 influenza A/H1N1 is caused by the neuraminidase mutation H275Y. This mutation has also been associated with in vitro resistance to peramivir, but few clinical cases have been described to date. Using allele-specific real-time reverse transcriptase polymerase chain reaction assay for the H275Y mutation, we were able to identify resistant H1N1 in a hematopoietic cell transplant recipient receiving intravenous peramivir therapy, and through serial testing we determined the molecular evolution of resistance. This case demonstrates that an H275Y mutant population can emerge early and replicate in vivo under peramivir antiviral pressure to become the major viral population.

Molecular studies of influenza B virus in the reverse genetics era

Recovery of an infectious virus of defined genetic structure entirely from cDNA and the deduction of information about the virus resulting from phenotypic characterization of the mutant is the process of reverse genetics. This approach has been possible for a number of negative-strand RNA viruses since the recovery of rabies virus in 1994. However, the recovery of recombinant orthomyxoviruses posed a greater challenge due to the segmented nature of the genome. It was not until 1999 that such a system was reported for influenza A viruses, but since that time our knowledge of influenza A virus biology has grown dramatically. Annual influenza epidemics are caused not only by influenza A viruses but also by influenza B viruses. In 2002, two groups reported the successful recovery of influenza B virus entirely from cDNA. This has allowed greater depth of study into the biology of these viruses. This review will highlight the advances made in various areas of influenza B virus biology as a result of the development of reverse genetics techniques for these viruses, including (i) the importance of the non-coding regions of the influenza B virus genome; (ii) the generation of novel vaccine strains; (iii) studies into the mechanisms of drug resistance; (iv) the function(s) of viral proteins, both those analogous to influenza A virus proteins and those unique to influenza B viruses. The information generated by the application of influenza B virus reverse genetics systems will continue to contribute to our improved surveillance and control of human influenza.

Characterization of oseltamivir-resistant 2009 H1N1 pandemic influenza A viruses

Influenza viruses resistant to antiviral drugs emerge frequently. Not surprisingly, the widespread treatment in many countries of patients infected with 2009 pandemic influenza A (H1N1) viruses with the neuraminidase (NA) inhibitors oseltamivir and zanamivir has led to the emergence of pandemic strains resistant to these drugs. Sporadic cases of pandemic influenza have been associated with mutant viruses possessing a histidine-to-tyrosine substitution at position 274 (H274Y) in the NA, a mutation known to be responsible for oseltamivir resistance. Here, we characterized in vitro and in vivo properties of two pairs of oseltaimivir-sensitive and -resistant (possessing the NA H274Y substitution) 2009 H1N1 pandemic viruses isolated in different parts of the world. An in vitro NA inhibition assay confirmed that the NA H274Y substitution confers oseltamivir resistance to 2009 H1N1 pandemic viruses. In mouse lungs, we found no significant difference in replication between oseltamivir-sensitive and -resistant viruses. In the lungs of mice treated with oseltamivir or even zanamivir, 2009 H1N1 pandemic viruses with the NA H274Y substitution replicated efficiently. Pathological analysis revealed that the pathogenicities of the oseltamivir-resistant viruses were comparable to those of their oseltamivir-sensitive counterparts in ferrets. Further, the oseltamivir-resistant viruses transmitted between ferrets as efficiently as their oseltamivir-sensitive counterparts. Collectively, these data indicate that oseltamivir-resistant 2009 H1N1 pandemic viruses with the NA H274Y substitution were comparable to their oseltamivir-sensitive counterparts in their pathogenicity and transmissibility in animal models. Our findings highlight the possibility that NA H274Y-possessing oseltamivir-resistant 2009 H1N1 pandemic viruses could supersede oseltamivir-sensitive viruses, as occurred with seasonal H1N1 viruses.

Evidence obtained from ANOVA to reason cross-species infection and cross-subtype mutation in neuraminidases of influenza A viruses

The current pandemic of A/H1N1 influenza raises a serious question on cross-species infection and cross-subtype mutation because our previous focus on possible influenza pandemic laid on H5N1 subtype and the cross-species infection between avian and human. In this study, we analyse 3874 neuraminidases from influenza A viruses using anova to answer the question of if there is barrier between species and between subtypes. The results show that there is no cross-species barrier in some species, and the intra-species variation is larger than the inter-species variation in some species hosting the viruses, and the cross-subtype mutation is possible because there is no cross-subtype barrier in some subtypes and the intra-subtype variation is larger than the inter-subtype variation in some subtypes. These results highlight the state of barrier of influenza A virus, which can help us understand the current pandemic and manufacture more effective vaccines.

Antiviral strategies for pandemic and seasonal influenza

While vaccines are the primary public health response to seasonal and pandemic flu, short of a universal vaccine there are inherent limitations to this approach. Antiviral drugs provide valuable alternative options for treatment and prophylaxis of influenza. Here, we will review drugs and drug candidates against influenza with an emphasis on the recent progress of a host-targeting entry-blocker drug candidate, DAS181, a sialidase fusion protein.

Course of seasonal influenza A/Brisbane/59/07 H1N1 infection in the ferret

Every year, influenza viruses infect approximately 5-20% of the population in the United States leading to over 200,000 hospitalizations and 36,000 deaths from flu-related complications. In this study, we characterized the immune and pathological progression of a seasonal strain of H1N1 influenza virus, A/Brisbane/59/2007 in a ferret model. The immune response of the animals showed a dose-dependent increase with increased virus challenge, as indicated by the presence of virus specific IgG, IgM, and neutralizing antibodies. Animals infected with higher doses of virus also experienced increasing severity of clinical symptoms and fever at 2 days post-infection (DPI). Interestingly, weight loss was more pronounced in animals infected with lower doses of virus compared to those infected with a higher dose; these results were consistent with viral titers of swabs collected from the nares, but not the throat. Analyzed specimens included nasal and throat swabs from 1, 3, 5, and 7 DPI as well as tissue samples from caudal lung and nasal turbinates. Viral titers of the swab samples in all groups were higher on 1 and 3 DPI and returned to baseline levels by 7 DPI. Analysis of nasal turbinates indicated presence of virus at 3 DPI in all infected groups, whereas virus was only detected in the lungs of animals in the two highest dose groups. Histological analysis of the lungs showed a range of pathology, such as chronic inflammation and bronchial epithelial hypertrophy. The results provided here offer important endpoints for preclinical testing of the efficacy of new antiviral compounds and experimental vaccines.

Protection from the 2009 H1N1 pandemic influenza by an antibody from combinatorial survivor-based libraries

Influenza viruses elude immune responses and antiviral chemotherapeutics through genetic drift and reassortment. As a result, the development of new strategies that attack a highly conserved viral function to prevent and/or treat influenza infection is being pursued. Such novel broadly acting antiviral therapies would be less susceptible to virus escape and provide a long lasting solution to the evolving virus challenge. Here we report the in vitro and in vivo activity of a human monoclonal antibody (A06) against two isolates of the 2009 H1N1 pandemic influenza virus. This antibody, which was obtained from a combinatorial library derived from a survivor of highly pathogenic H5N1 infection, neutralizes H5N1, seasonal H1N1 and 2009 “Swine” H1N1 pandemic influenza in vitro with similar potency and is capable of preventing and treating 2009 H1N1 influenza infection in murine models of disease. These results demonstrate broad activity of the A06 antibody and its utility as an anti-influenza treatment option, even against newly evolved influenza strains to which there is limited immunity in the general population.

Influenza viruses constantly challenge our ability to prevent and treat their resulting infection. From a survivor of the H5N1 influenza we have discovered an antibody that is effective against both H5N1 and seasonal H1N1 influenza viruses. Here we show the antibody is effective against 2009 pandemic influenza in a cell culture assay and also in mouse models of disease when given before and even after lethal influenza infection. The present work demonstrates the viability of this particular antibody and the general approach of using antibodies against viral pathogens as opposed to traditional treatments that are losing their efficacy for the prevention and treatment of influenza infection. We conclude the efficacy of this antibody warrants further experimental testing as an alternative therapy for treatment in man.

Identification of influenza A nucleoprotein as an antiviral target

The spread of influenza virus strains resistant to the current generation of anti-viral drugs makes the identification of new druggable targets and lead compounds of prime importance. Kao et al. show that the influenza A nucleoprotein can be targeted by a small molecule that protects mice from lethal viral challenges.

Influenza A remains a significant public health challenge because of the emergence of antigenically shifted or highly virulent strains^1,2,3,4,5. Antiviral resistance to available drugs such as adamantanes or neuraminidase inhibitors has appeared rapidly^6,7,8,9, creating a need for new antiviral targets and new drugs for influenza virus infections. Using forward chemical genetics, we have identified influenza A nucleoprotein (NP) as a druggable target and found a small-molecule compound, nucleozin, that triggers the aggregation of NP and inhibits its nuclear accumulation. Nucleozin impeded influenza A virus replication in vitro with a nanomolar median effective concentration (EC₅₀) and protected mice challenged with lethal doses of avian influenza A H5N1. Our results demonstrate that viral NP is a valid target for the development of small-molecule therapies.

A new antigenic variant of human influenza A (H3N2) virus isolated from airport and community surveillance in Taiwan in early 2009

A new variant of influenza A H3N2 virus emerged in January 2009 and became the dominant strain in Taiwan in April 2009. The variant was also detected in imported cases from various regions, including East and Southeast Asia and North America, indicating that it has circulated globally. Compared to the 2009-2010 vaccine strain, A/Brisbane/10/2007, the hemagglutinin gene of this variant exhibited five substitutions, E62K, N144K, K158N, K173Q and N189K, which are located in the antigenic sites E, A, B, D and B respectively, and it was antigenically distinct from A/Brisbane/10/2007 with more than eight-fold titer reduction in the hemagglutination inhibition reaction. The A/Perth/16/2009 (H3N2)-like virus recommended by World Health Organization for use in the 2010 southern hemisphere and 2010-2011 northern influenza seasons exhibited the same substitutions like this new variant. In addition to regional or community influenza surveillance, the imported cases or airport fever screening surveillance may be a good resource to monitor the evolution of the virus and benefit the real-time information of global influenza circulation.

T-705 (favipiravir) activity against lethal H5N1 influenza A viruses

The neuraminidase inhibitors oseltamivir and zanamivi are used to treat H5N1 influenza. However, oseltamivir-resistant H5N1 viruses have been isolated from oseltamivir-treated patients. Moreover, reassortment between H5N1 viruses and oseltamvir-resistant human H1N1 viruses currently circulating could create oseltamivir-resistant H5N1 viruses, rendering the oseltamivir stockpile obsolete. Therefore, there is a need for unique and effective antivirals to combat H5N1 influenza viruses. The investigational drug T-705 (favipiravir; 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) has antiviral activity against seasonal influenza viruses and a mouse-adapted H5N1 influenza virus derived from a benign duck virus. However, its efficacy against highly pathogenic H5N1 viruses, which are substantially more virulent, remains unclear. Here, we demonstrate that T-705 effectively protects mice from lethal infection with oseltamivir-sensitive or -resistant highly pathogenic H5N1 viruses. Furthermore, our biochemical analysis suggests that T-705 ribofuranosyl triphosphate, an active form of T-705, acts like purines or purine nucleosides in human cells and does not inhibit human DNA synthesis. We conclude that T-705 shows promise as a therapeutic agent for the treatment of highly pathogenic H5N1 influenza patients.

Synthesis and anti-viral activity of azolo-adamantanes against influenza A virus

Chemotherapy and chemoprophylaxis of influenza is one of the most important directions of health protection activity. Due to the high rate of drug-resistant strains of influenza virus, there is a need for the search and further development of new potent antivirals against influenza with a broad spectrum of activity. In the present study, a set of di-, tri- and tetrazole derivatives of adamantane was efficiently prepared and their anti-influenza activities evaluated against rimantadine-resistant strain A/Puerto Rico/8/34. In general, derivatives of tetrazole possessed the highest virus-inhibiting activity. We demonstrated that several compounds of this set exhibited much higher activity than the currently used antiviral rimantadine, a compound of related structure. Moreover, we showed that these azolo-adamantanes were significantly less toxic. This study demonstrates that influenza viruses can be inhibited by adamantyl-azoles and thus have potential for developing antiviral agents with an alternate mechanism of action.

Influenza, respiratory syncytial virus and SARS

Acute lower respiratory tract infections (LRTIs) are a major worldwide health problem, particularly in childhood. About 30-50% of acute LRTIs are viral in origin; of these, influenza and respiratory syncytial virus are associated with the greatest disease burden in humans. Many different influenza A viruses occur naturally in animal reservoirs, and present a constant threat of zoonotic infections and global pandemics. The pandemic (H1N1) influenza virus that emerged in humans in 2009 contained a unique combination of genes originating in swine and the global human population was highly susceptible to the novel strain. The emergence of the severe acute respiratory syndrome coronavirus in 2003, and the ensuing worldwide epidemic, highlights the fact that respiratory viral infections in humans may originate in animals. Preventative measures for influenza include annual vaccination and treatment with antiviral drugs such as the neuraminidase inhibitors oseltamivir and zanamivir. Subtype-dependent resistance to antivirals can develop and should be closely monitored.

Anti-viral properties and mode of action of standardized Echinacea purpurea extract against highly pathogenic avian influenza virus (H5N1, H7N7) and swine-origin H1N1 (S-OIV)

Background

Influenza virus (IV) infections are a major threat to human welfare and animal health worldwide. Anti-viral therapy includes vaccines and a few anti-viral drugs. However vaccines are not always available in time, as demonstrated by the emergence of the new 2009 H1N1-type pandemic strain of swine origin (S-OIV) in April 2009, and the acquisition of resistance to neuraminidase inhibitors such as Tamiflu^® (oseltamivir) is a potential problem. Therefore the prospects for the control of IV by existing anti-viral drugs are limited. As an alternative approach to the common anti-virals we studied in more detail a commercial standardized extract of the widely used herb Echinacea purpurea (Echinaforce^®, EF) in order to elucidate the nature of its anti-IV activity.

Results

Human H1N1-type IV, highly pathogenic avian IV (HPAIV) of the H5- and H7-types, as well as swine origin IV (S-OIV, H1N1), were all inactivated in cell culture assays by the EF preparation at concentrations ranging from the recommended dose for oral consumption to several orders of magnitude lower. Detailed studies with the H5N1 HPAIV strain indicated that direct contact between EF and virus was required, prior to infection, in order to obtain maximum inhibition in virus replication. Hemagglutination assays showed that the extract inhibited the receptor binding activity of the virus, suggesting that the extract interferes with the viral entry into cells. In sequential passage studies under treatment in cell culture with the H5N1 virus no EF-resistant variants emerged, in contrast to Tamiflu^®, which produced resistant viruses upon passaging. Furthermore, the Tamiflu^®-resistant virus was just as susceptible to EF as the wild type virus.

Conclusion

As a result of these investigations, we believe that this standard Echinacea preparation, used at the recommended dose for oral consumption, could be a useful, readily available and affordable addition to existing control options for IV replication and dissemination.

Evaluation of a rapid molecular algorithm for detection of pandemic influenza A (H1N1) 2009 virus and screening for a key oseltamivir resistance (H275Y) substitution in neuraminidase

BACKGROUND

Rapid and specific molecular tests for identification of the recently identified pandemic influenza A/H1N1 2009 virus as well as rapid molecular tests to identify antiviral resistant strains are urgently needed.

OBJECTIVES

We have evaluated the performance of two novel reverse transcriptase polymerase chain reactions (RT-PCRs) targeting specifically hemagglutinin and neuraminidase of pandemic influenza A/H1N1 virus in combination with a conserved matrix PCR. In addition, we investigated the performance of a novel discrimination RT-PCR for detection of the H275Y resistance mutation in the neuraminidase gene.

STUDY DESIGN

Clinical performance of both subtype specific RT-PCR assays was evaluated through analysis of 684 throat swaps collected from individuals meeting the WHO case definition for the novel pandemic influenza virus. Analytical performance was analyzed through testing of 10-fold serial dilutions of RNA derived from the first Dutch sequenced and cultured confirmed case of novel pandemic influenza infection. Specificity and discriminative capacities of the H275Y discrimination assay were performed by testing wild type and recombinant H275Y pandemic influenza.

RESULTS

121 throat swaps collected from April 2009 to July 2009 were positive by at least two out of three RT-PCRs, and negative for the seasonal H3/H1 subtype specific RT-PCR assays. 117 of these were tested positive for all three (Ct-values from 15.1 to 36.8). No oseltamivir resistance was detected.

CONCLUSIONS

We present a sensitive and specific approach for detection of pandemic influenza A/H1N1 2009 and a rapid RT-PCR assay detecting a primary oseltamivir resistance mutation which can be incorporated easily into clinical virology algorithms.

A(H5N1) Virus Evolution in South East Asia

Highly Pathogenic Avian Influenza (HPAI) H5N1 virus is an ongoing public health and socio-economic challenge, particularly in South East Asia. H5N1 is now endemic in poultry in many countries, and represents a major pandemic threat. Here, we describe the evolution of H5N1 virus in South East Asia, the reassortment events leading to high genetic diversity in the region, and factors responsible for virus spread. The virus has evolved with genetic variations affecting virulence, drug-resistance, and adaptation to new host species. The constant surveillance of these changes is of primary importance in the global efforts of the scientific community.

Extraction of catalytically active neuraminidase of H5N1 influenza virus using thrombin proteolytic cleavage

The stalk of influenza neuraminidase (NA) has been a target of cleavage by various proteases, resulting in the release of catalytically active globular heads from virus particles. However, despite successful cases in a number of influenza subtypes, this strategy could not be applied to all influenza viruses due to high variation of the NA stalk. In the present study, reverse genetics was employed to construct non-pathogenic recombinant influenza A viruses, termed rgH1N1(LVPR) and rgH1N1(LVPR-GS), that harbor the NA of H5N1 virus engineered to contain a specific thrombin cleavage site at the stalk region. By using thrombin to cleave NA at its stalk, a productive extraction of NA globular heads could be obtained from purified rgH1N1(LVPR). Furthermore, it was found that the NA of rgH1N1(LVPR-GS) could be cleaved by endogenous thrombin present in embryonated chicken eggs, resulting in the release of NA globular heads into allantoic fluids. These data highlight the use of thrombin cleavage as an effective strategy for extraction of active NA heads directly from live viral particles not only of H5N1 but, theoretically, of any subtype of influenza A viruses.

Large-scale sequence analysis of M gene of influenza A viruses from different species: mechanisms for emergence and spread of amantadine resistance

Influenza A virus infects many species, and amantadine is used as an antiviral agent. Recently, a substantial increase in amantadine-resistant strains has been reported, most of which have a substitution at amino acid position 31 in the M2 gene. Understanding the mechanism responsible for the emergence and spread of antiviral resistance is important for developing a treatment protocol for seasonal influenza and for deciding on a policy for antiviral stockpiling for pandemic influenza. The present study was conducted to identify the existence of drug pressure on the emergence and spread of amantadine-resistant influenza A viruses. We analyzed data on more than 5,000 virus sequences and constructed a phylogenetic tree to calculate selective pressures on sites in the M2 gene associated with amantadine resistance (positions 26, 27, 30, and 31) among different hosts. The phylogenetic tree revealed that the emergence and spread of the drug-resistant M gene in different hosts and subtypes were independent and not through reassortment. For human influenza virus, positive selection was detected only at position 27. Selective pressures on the sites were not always higher for human influenza virus than for viruses of other hosts. Additionally, selective pressure on position 31 did not increase after the introduction of amantadine. Although there is a possibility of drug pressure on human influenza virus, we could not find positive pressure on position 31. Because the recent rapid increase in drug-resistant virus is associated with the substitution at position 31, the resistance may not be related to drug use.

The origin and global emergence of adamantane resistant A/H3N2 influenza viruses

Resistance to the adamantane class of antiviral drugs by human A/H3N2 influenza viruses currently exceeds 90% in the United States and multiple Asian countries. Adamantane resistance is associated with a single amino acid change (S31N) in the M2 protein, which was shown to rapidly disseminate globally in 2005 in association with a genome reassortment event. However, the exact origin of influenza A/H3N2 viruses carrying the S31N mutation has not been characterized, particularly in South-East Asia. We therefore conducted a phylogenetic analysis of the HA, NA, and M1/2 segments of viral isolates collected between 1997 and 2007 from temperate localities in the Northern hemisphere (New York State, United States, 492 isolates) and Southern hemisphere (New Zealand and Australia, 629 isolates) and a subtropical locality in South-East Asia (Hong Kong, 281 isolates). We find that although the S31N mutation was independently introduced at least 11 times, the vast majority of resistant viruses now circulating globally descend from a single introduction that was first detected in the summer of 2003 in Hong Kong. These resistant viruses were continually detected in Hong Kong throughout 2003-2005, acquired a novel HA through reassortment during the first part of 2005, and thereafter spread globally. The emergence and persistence of adamantane resistant viruses in Hong Kong further supports a source-sink model of global influenza virus ecology, in which South-East Asia experiences continuous viral activity and repeatedly seeds epidemics in temperate areas.

Seasonal influenza in adults and children--diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America

Guidelines for the treatment of persons with influenza virus infection were prepared by an Expert Panel of the Infectious Diseases Society of America. The evidence-based guidelines encompass diagnostic issues, treatment and chemoprophylaxis with antiviral medications, and issues related to institutional outbreak management for seasonal (interpandemic) influenza. They are intended for use by physicians in all medical specialties with direct patient care, because influenza virus infection is common in communities during influenza season and may be encountered by practitioners caring for a wide variety of patients.

Antiviral Resistance in Influenza Viruses: Clinical and Epidemiological Aspects

Two classes of anti-viral agents, the M2 ion channel inhibitors (amantadine, rimantadine) and neuraminidase (NA) inhibitors (oseltamivir, zanamivir) are available for treatment and prevention of infl uenza in most countries of the world. The principle concerns about emergence of antiviral resistance in infl uenza viruses are loss of drug effi cacy, transmission of resistant variants, and possible increased virulence or transmissibility of resistant variants (1). Because seasonal infl uenza is usually an acute, self-limited illness in which viral clearance occurs rapidly due to innate and adaptive host immune responses, the emergence of drug-resistant variants would be anticipated to have modest effects on clinical recovery, except perhaps in immunocompromised or immunologically naïve hosts, such as young infants or during the appearance of a novel strain. In contrast to the limited impact of resistance emergence in the treated immunocompetent individual, the epidemiologic impact of resistance emergence and transmission could be considerable, including loss of both prophylactic and therapeutic activity for a particular drug, at the household, community, or perhaps global level. Infl uenza epidemiology in temperate climates is expected to provide some protection against widespread circulation of resistant variants, as viruses do not persist between epidemics but rather are re-introduced each season and new variants appear often (2, 3).

Surveillance for neuraminidase-inhibitor-resistant influenza viruses in Japan, 1996–2007

Background

High usage of the neuraminidase inhibitor (NAI) oseltamivir in Japan since 2003 led the Neuraminidase Inhibitor Susceptibility Network to assess the susceptibility of community isolates of influenza viruses to oseltamivir and zanamivir.

Methods

Isolates were tested by the enzyme inhibition assay and by neuraminidase (NA) sequence analysis.

Results

Among 1,141 A(H3N2) viruses and 171 type B viruses collected in Japan during the 2003–2004 season, 3 (0.3%) A(H3N2) isolates showed high 50% inhibitory concentrations (IC50) to oseltamivir. Each possessed a known resistance NA mutation at R292K or E119V. During the 2004–2005 season, no resistance was found among 567 influenza A(H3N2) or 60 A(H1N1) isolates, but 1 of 58 influenza B isolates had an NAI resistance mutation (D197N). Sequence analysis found that 4 (3%) of 132 A(H1N1) viruses from 2005–2006 had known NA resistance mutations (all H274Y), but no additional resistant isolates were detected from that or the subsequent 2006–2007 season. Concurrent testing of a selection of 500 influenza B viruses from 2000 to 2006 showed significant variations between seasons in both oseltamivir and zanamivir IC50 values, but no persistent increases over this period.

Conclusions

Our findings suggested possible low-level transmission of resistant variants from oseltamivir-treated patients in several seasons in Japan but no sustained reductions in NAI susceptibility or consistently increased frequency of detecting resistant variants for any strain or subtype, despite high levels of drug use. In particular, although oseltamivir-resistant A(H1N1) viruses with the H274Y mutation spread globally in 2007–2008, we found little evidence for increasing levels of resistant A(H1N1) variants in Japan in preceding years.