Prevalence And Biological Fitness Of Influenza B Viruses With Drug-resistance During2008-2012Influenza Seasons In China

Background:With the emergence and wide spread of the oseltamivir-resistant seasonal influenza A(H1N1) virus in2007, it aroused great concern among nations. Therefore, it seems to be particularly necessary to develop drug-resistant surveillance. According to the surveillance reports, almost all of the influenza A viruses have been resistant to adamantanes, but most of the viruses circulating now remain sensitive to neuraminidase inhibitors (NAIs). Nevertheless, very little is known about the biological fitness of influenza B viruses with drug resistance or reduced sensitivity to neuraminidase inhibitors.Objectives:To study the susceptibility of influenza B viruses to NAIs in China, and to study the infectivity and transmissibility patterns of influenza B viruses with drug resistance or reduced sensitivity to NAIs in China, to provide clues for the identification of secondary compensatory mutations contributing to the influenza B viruses with drug resistance or reduced sensitivity.Methods:In this study,682influenza B viruses isolated from ILI (Influenza like illness) cases during2008-2012influenza seasons in China were analyzed for resistance to NAIs, by genetic analysis and/or chemiluminescent NA activity inhibition assay. The virus that proved to be resistant or with reduced sensitivity to NAIs and its wild-type sensitive virus were selected and plaque purified for3rounds. The replicative fitness, infectivity and transmissibility of these viruses were further studied both in vitro (MDCK cell) and in vivo (ferret)Results:6variants with resistance or reduced sensitivity to NAIs were identified. Compared to WT virus(B/SichuanYucheng/1238/2010, wild type virus), the MUT virus(B/Shanghai-Pudongxi/1125/2011,mutant virus) which carried NA I221T mutation associated with drug-resistance and HA S208P secondary mutation, had reduced sensitivity against oseltamivir(9fold increase for IC50value). In addition, the NA Km value of MUT were lower than that of WT, and the replication kinetic of MUT was higher than that of WT in vitro, but these difference had no statistical significance. And in vivo, both MUT and WT could spread effectively among ferrets by close contact. And MUT’ transmissibility was slightly weaker than WT. To sum up, the biological fitness of MUT in vitro and vivo are both similar with that of WT. The NA I221T mutation may not influence the biological fitness of viruses. Moreover, the secondary mutation S208P in HA of MUT may contribute to the biological fitness, but further study will be needed to confirm this.Conclusions:In a word, all these highlighted the importance of sustained national and even global monitoring for antiviral resistance among circulating influenza B viruses to public health and clinical recommendations for antivirals.