Analysis of epidemiological characteristics of influenza in Nanjing, from 2009 to 2013

Objective To describe the epidemiological pattern and characteristics of influenza in Nanjing for the period from 2009 to 2013. Methods Influenza samples were collected from influenza like illness (ILI) patients, the positive cases and influenza subtype were determined by reverse transcriptase polymerase chain reaction. Excel 2007 and SPSS 19.0 were used to analyze the data. Results The influenza positive rate for the period from 2009 to 2013 in Nanjing was 16.7% with 2 990 positives out of a total of 17 906 samples. The positive rate varied by month, quarter (season), and year describing a sequence of peaks presenting primarily in autumn and winter with some summer peak periods. Influenza A was dominant in summer and autumn while B was dominant in winter. The Dominant subtypes in Nanjing in 2009-2013 were 2009 pandemic H1N1, H3N2, and influenza B. H3N2 generally peaked in autumn or summer, 2009 pandemic H1N1 peaked in autumn, and influenza B mainly occurred in winter. The Nanjing influenza season peaks were mostly in autumn and early winter, but also in some years, summer and early autumn peaks of H3N2. Standard modeling using the moving epidemic method showed that the typical or nonpandemic influenza season in Nanjing lasted approximately 11 weeks and had an epidemic threshold of 25.21% and a background rate of 5.82% positive cases of influenza. Conclusions The peak influenza season in Nanjing usually occurs once a year but in some years there may be two peak periods of influenza activity.