Spatial timing of circulating seasonal influenza A and B viruses in China from 2014 to 2018.

ABSTRACT

Major outbreaks of influenza virus occurred in China in 2017-2018. To describe the pattern of influenza circulation and timing of seasonal epidemics, we analyzed data from influenza-like illness (ILI) specimens on surveillance wards of sentinel hospitals during 2014-2018. Among 1,890,084 ILI cases, 324,211 (17.2%) tested positive for influenza. Influenza A virus (particularly A/H3N2), which circulates annually, was detected in 62% of cases, compared with influenza B virus in 38% of cases. The detection rate of A/H1N1, A/H3N2, B/Victoria, and B/Yamagata viruses were 3.56%, 7.07%, 2.08%, and 3.45%, respectively. Influenza prevalence was generally stable over the four years analyzed, but obvious outbreaks occurred in 2015-2016 (17.28%) and 2017-2018 (22.67%), with B/Victoria and B/Yamagata contributing to these outbreaks, respectively. In the south, a characteristic peak in infections was detected in the summer (week 23-38), which was not detected in the north. Influenza B was found high frequency in school-age children (5-14 years) with 4.78% of B/Victoria and 6.76% of B/Yamagata. Therefore, the epidemiological characteristics of seasonal influenza were complex in China during 2014-2018, presenting distinctions in region, season, and susceptible population. These findings underline the importance of enhancing year-round influenza surveillance and provide a reference for the timing and variety of influenza vaccination.