Epidemiology of human infections with seasonal influenza B in mainland China, 2005-2014

Abstract

BACKGROUND: Seasonal influenza viruses have caused considerable disease burden in humans every year. Substantial contribution from influenza B to the overall impact of influenza in a population has been noted. Currently there are limited data on systematic investigation of human infections with influenza B virus. We used sentinel surveillance data in mainland China to examine the epidemiology of influenza B virus infection in the population. METHOD: Influenza surveillance data were obtained from 556 sentinel hospitals in mainland China from 2005 to 2014. Specimens collected from outpatients with influenza-like-illness were tested for influenza viruses. Virus lineages were determined by PCR and/or hemagglutination inhibition test and verified by the Chinese National Influenza Center. Individual data on age, sex, date of symptom onset and virologic results were obtained for all patients selected at sentinel sites. Analyses were conducted to compare epidemiologic characteristics, seasonality and geographic distributions between influenza B Victoria and Yamagata lineages. RESULTS: Influenza B viruses circulated every year in mainland China with 15-20% sentinel specimens testing positive during peak seasons in 2005-2014,. The Victoria lineage was predominent in most winter seasons except for 2007- 08 and 2013-14. Both lineages were frequently observed in winter-spring while B Yamagata circulated largely year-round especially in southern provinces. Influenza B infections were more common in children than adults, and children less than 10 years had the highest frequency of infections compared to other age groups while Victoria lineage was likely to infect more young adults than B Yamagata. CONCLUSION: Influenza B viruses caused seasonal epidemics in humans every year. Potential differences were observed in seasonality, geographic distribution and age patterns of sentinel cases infected with Victoria and Yamagata lineages.