Analysis on influenza surveillance in Zhongshan city from 2009 to 2012

Objective To analyse the results of influenza surveillance in Zhongshan city from 2009 to 2012 ,to understand its epidemiological characteristics and virus type ,and provide scientific evidence for developing influenza prevention and control strategy. Methods Influenza surveillance system in Zhongshan was consisted of influenza-like-illness (ILI) surveillance,etiological surveillance and outbreak monitoring surveillance ,ILI% (ILI% =outpatient and emergency ILI visits / outpatient and emergency total visits×100% )and the positive rate of influenza virus were the main analysis indexes. Results Influenza was active in 2009,with two peaks of clinic visits for ILI, and the first peak was caused by influenza A(H3N2), while the second one was caused by the new influenza A (H1N1). After the influenza pandemic in 2009 , the epidemic of influenza in 2010 declined rapidly and remained at a relatively stable state. Influenza B virus was the major strain in the first half of the year,while A (H3N2)was the major strain in the second half of the year.From 2011 to 2012, the new influenza A (H1N1),influenza B virus and influenza A (H3N2) subtype were alternately as dominant strains,and every year the peaks of clinic visits for ILI occurred in spring and summer.Spearman correlation analysis indicated that ILI rate and positive rate of etiology monitoring was positively correlated (r=0.701,P0.01).From 2009 to 2012,a total of 11 outbreaks with 580 cases were reported ,which mainly occurred in primary and middle schools and the average attack rate was 1.25%. Conclusion From 2009 to 2012, the predominant strains changed regularly.The ILI epidemic strength was correlated with the influenza virus activity.