Analysis of influenza surveillance in Haidian District of Beijing, 2011-2015

Abstract

Objective To analyze epidemic characteristics of influenza virus in Haidian District of Beijing from 2011 to 2015 and to provide scientific basis for influenza prevention and control. Methods The numbers of influenza-like illness(ILI) cases reported by 63 sentinel hospitals of Haidian district from 2011 to 2015 and the detection results of etiological surveillance samples collected from one national influenza surveillance hospital from Sep 2011 to Aug 2015 were used to analyze. Results Among 49 228 815 cases in outpatient and emergency in 63 sentinel hospitals from 2011 to 2015, the ILI count and rate were 362 410 and 0.74%, respectively. Proportions of ILI(ILI%) in all surveillance cases were different in 5 years(χ2=1 103.798, P<0.05). Age group compositions of ILI varied from 2011 to 2015(P<0.05). From Sep 2011 to Aug 2015, 4 183 throat swab specimens were collected from ILI cases during 4 influenza etiological surveillance seasons and 766 specimens were tested positive for influenza. The total positive rate was 18.31%. Positive rates of influenza virus nucleic acid detection in the season during 2013- 2014 were different during 2014-2015(χ2=6.955, P=0.008). And subtype compositions of influenza virus varied in different surveillance seasons from 2011 to 2015(P<0.05). Weekly positive rates were positively related to weekly numbers of ILI cases and weekly ILI%(r=0.579, P<0.05; r=0.602, P<0.05, respectively). Conclusions There were certain rules of influenza epidemic characteristics in Haidian District that the peak onset of seasonal flu was from the 50th week (December) to 16th week (April) in the next year. Influenza epidemic was more active in the season from 2014 to 2015 than in previous years, and the subtypes of dominate strain was A (H3N2). The dominate influenza virus subtypes varied in different years with the feature that decline of one dominate subtype accompanied with growth of another one in one year. Key words: Influenza; Influenza-like illness (ILI); Epidemic characteristics