BACKGROUND: During the 2014-15 US influenza season, expanded genetic characterization of circulating influenza A(H3N2) viruses was used to assess the impact of genetic variability of influenza A(H3N2) viruses on influenza vaccine effectiveness (VE). METHODS: A novel pyrosequencing assay was used to determine genetic group based on hemagglutinin (HA) gene sequences of influenza A(H3N2) viruses from patients enrolled US Flu Vaccine Effectiveness network sites. Vaccine effectiveness was estimated using a test-negative design comparing vaccination among patients infected with influenza A(H3N2) viruses and uninfected patients. RESULTS: Among 9710 enrollees, 1868 (19%) tested positive for influenza A(H3N2); genetic characterization of 1397 viruses showed 1134 (81%) belonged to one HA genetic group (3C.2a) of antigenically drifted H3N2 viruses. Effectiveness of 2014-15 influenza vaccination varied by A(H3N2) genetic group from 1% (95% confidence interval [CI], -14% to 14%) against illness caused by antigenically drifted A(H3N2) group 3C.2a viruses versus 44% (95% CI, 16% to 63%) against illness caused by vaccine-like A(H3N2) group 3C.3b viruses. CONCLUSION: Effectiveness of 2014-15 influenza vaccination varied by genetic group of influenza A(H3N2) virus. Changes in hemagglutinin genes related to antigenic drift were associated with reduced vaccine effectiveness.

Human infections by avian influenza A(H7N9) virus entail substantial morbidity and mortality. Treatment of infected patients with the neuraminidase (NA) inhibitor oseltamivir was associated with emergence of viruses carrying NA substitutions. In the NA inhibition (NI) assay, R292K conferred highly reduced inhibition by oseltamivir, while E119V and I222K each caused reduced inhibition. To facilitate establishment of laboratory correlates of clinically relevant resistance, experiments were conducted in ferrets infected with virus carrying wild-type or variant NA genes recovered from the A/Taiwan/1/2013 isolate. Oseltamivir treatment (5 or 25 mg/kg/dose) was given 4 hours post-infection followed by twice daily treatment for 5 days. Treatment of ferrets infected with wild-type virus resulted in a modest dose-dependent reduction (0.7-1.5 log10TCID50) in nasal wash viral titers and inflammation response. Conversely, treatment failed to significantly inhibit the replication of R292K or E119V viruses. A small reduction of viral titers was detected on day 5 in ferrets infected with the I222K virus. Propensity for oseltamivir resistance emergence was assessed in oseltamivir-treated animals infected with wild-type virus; emergence of R292K virus was detected in 3 of 6 ferrets within 5-7 days post-infection. Collectively, we demonstrate that R292K, E119V and I222K reduced the inhibitory activity of oseltamivir not only in the NI assay, but also in infected ferrets, judged particularly by viral loads in nasal washes, and may signal the need for alternative therapeutics. Thus, these clinical outcomes measured in the ferret model may correlate with clinically relevant oseltamivir resistance in humans. IMPORTANCE: This report provides more evidence for using the ferret model to assess susceptibility of influenza A(H7N9) viruses to oseltamivir, the most prescribed anti-influenza drug. The information gained can be used to assist in the establishment of laboratory correlates of human disease and drug therapy. The rapid emergence of viruses with R292K in treated ferrets correlates well with the multiple reports on this NA variant in treated human patients. Our findings highlight the importance for discovery and characterization of new antiviral drugs with different mechanism of action and the use of combination treatment strategies against emerging viruses with pandemic potential such as avian H7N9 virus, particularly against those carrying drug resistance markers.

BACKGROUND: Patients contracting influenza A(H7N9) often developed severe disease causing respiratory failure. Neuraminidase (NA) inhibitors (NAIs) are the primary option for treatment, but information on drug-resistance markers for A(H7N9) is limited. METHODS: Four NA variants of A/Taiwan/1/2013 (H7N9) virus containing a single substitution (NA-E119 V, NA-I222 K, NA-I222R or NA-R292 K), recovered from an oseltamivir-treated patient, were tested for NAI susceptibility in vitro; their replicative fitness was evaluated in cell culture, mice and ferrets. RESULTS: NA-R292 K led to highly reduced inhibition by oseltamivir and peramivir, while NA-E119 V, NA-I222 K and NA-I222R caused reduced inhibition by oseltamivir. Mice infected with any virus showed severe clinical signs with high mortality rates. NA-I222 K virus was the most virulent in mice, whereas virus lacking NA change (NA-WT) and NA-R292 K virus seemed the least virulent. Sequence analysis suggests that PB2-S714N increased virulence of the NA-I222 K virus in mice; NS1-K126R, alone or in combination with PB2-V227M, produced contrasting effects in NA-WT and NA-R292 K viruses. In ferrets, all viruses replicated to high titers in the upper respiratory tract, but produced only mild illness. NA-R292 K virus, showed reduced replicative fitness in this animal model. CONCLUSIONS: Our data highlight challenges in assessment of replicative fitness of H7N9 NA variants emerged in NAI-treated patients.

Human infections caused by the avian influenza A(H7N9) virus have been associated with substantial morbidity and mortality. Emergence of virus variants, carrying markers of decreased susceptibility to neuraminidase inhibitors, was reported. Here we showed that fludase, an antiviral drug with sialidase activity, potently inhibited replication of wild-type H7N9 viruses and their oseltamivir-resistant R292 K variants in mice. A once-daily administration initiated early after lethal infection, hampered body weight loss and completely protected mice from lethality. We observed a time-dependent effect for 24-72-hour delayed fludase treatments on morbidity and mortality. The results warrant further investigation of fludase for influenza treatment.