Epidemiological analysis of surveillance for influenzain Chaoyang District, Beijing, 2015-2016

Objective To analyze the epidemiological characteristics of influenza in Chaoyang District in Beijing from 2015 to 2016, so as to provide specific evidence for making preventive and control strategies of influenza. Methods Weekly data were collected from influenza-like illness (ILI) report, pathogen surveillance and influenza outbreak report in Chaoyang District in Beijing. Results From 2015 to 2016, a total of 187 072 ILI were reported from the sentinel hospitals in Chaoyang District, with the average of ILI% was 2.42%. The majority of cases were reported fromage group of 0-4, accounted for 44.78% of the total ILI. There was one peak in winter, and the peak of ILI and ILI% emerged in the 6th week and the 7th week in 2016 respectively. A total of 2 082 swab samples from ILI patients were collected and tested, from which 397 specimens were RNA positive, with a positive rate of 19.07%. Two marked epidemic peaks appeared from the 31st week to the 40th week and from the 51st week to the next 13th week, and the second one was more distinct. During this year, four subtypes of influenza virus were observed, and B Yamagata lineage virus, A(H3N2) and B Victoria lineage virus appeared as the dominating strains alternately. From 2015 to 2016, 43 cluster events of influenza were reported totally, and 42(97.67%) of them occurred in childcare facilities and primary school. There were positive correlations both between ILI%, ILI counts and the positive rate of influenza(r_s=0.644,P<0.001; r_s=0.600,P<0.001). Conclusions Different virus sub-types prevailed alternatively from 2015 to 2016. There were similar characteristics among ILI, pathogen surveillance and cluster events of influenza. Enhanced and continuous surveillance is important for the prevention and control of influenza.