Seroprevalence of pandemic (H1N1) 2009 influenza and effectiveness of 2010/2011 influenza vaccine during 2010/2011 season in Beijing, China

Please cite this paper as: Yang et al. (2011) Seroprevalence of pandemic (H1N1) 2009 influenza and effectiveness of 2010/2011 influenza vaccine during 2010/2011 season in Beijing, China. Influenza and Other Respiratory Viruses 6(6), 381–388. Background In the post‐pandemic period, pandemic (H1N1) 2009 virus was expected to circulate seasonally and was introduced into trivalent influenza vaccine during 2010/2011 season in the Northern Hemisphere. Objectives The aim of this study was to examine the evolution of herd immunity against pandemic (H1N1) 2009 virus in Beijing, China, during 2010/2011 season and effectiveness of the 2010/2011 trivalent vaccine. Methods Two serological surveys were conducted before and after 2010/2011 season in Beijing. A case–control study was used to investigate vaccine effectiveness against influenza‐like illness (ILI) and lower respiratory tract infection (LRI). Results A total of 4509 and 4543 subjects participated in the pre‐ and post‐season surveys, respectively. The standardized seroprevalence of pandemic (H1N1) 2009 influenza increased from 22·1% pre‐season to 24·3% post‐season (P < 0·001). Significant elevation in seroprevalence appeared in the ≥60 years age‐group (P < 0·001), but not in others. The 2010/2011 trivalent vaccine contributed to the higher post‐seasonal seroprevalence in unvaccinated individuals (P = 0·024), but not in those vaccinated with monovalent pandemic vaccine (P = 0·205), as well as in those without prior immunity versus those with immunity. The adjusted effectiveness of the 2010/2011 trivalent vaccine was 79% protection against ILI (95% CI, 61–89%) and 95% against LRI (95% CI: 59–99%). Conclusions A slight increase in herd immunity against pandemic (H1N1) 2009 influenza was observed in Beijing, China, during the 2010/2011 season. Prior vaccination and immunity had a suppressive impact on immune response toward this novel influenza virus, elicited by 2010/2011 trivalent vaccine. This trivalent vaccine conferred good protection against ILI and LRI.