Objective  To investigate the epidemiological characteristics of influenza in children in Zhoushan, Zhejiang during the influenza seasons from week 40, 2016 to week 13, 2020. Through calculating the epidemic thresholds and intensity thresholds of the influenza in children based on the moving epidemic method (MEM), to provide early-warning information for preventing influenza infection of local children.

  Methods  The pediatric outpatient setting and emergency department visits of children in sentinel hospital in Zhoushan from week 40, 2016 to week 13, 2020 were recorded, and the percentages of visits for influenza-like illness per week (ILI%) were calculated. The positive rates of influenza virus detection per week were calculated, which alongside ILI%, were considered as the monitoring indicators of influenza activity level. Excel 2007 and SPSS 23.0 were used for statistical processing. In addition, datas of positive rate of influenza virus detection per week in Zhoushan from 2016 to 2019 were used as historical datas for generating the MEM model, and thus the pre-epidemic threshold, the post-epidemic threshold and the intensity thresholds of the epidemic season in Zhoushan from 2019 to 2020 were calculated.

  Results  A total of 21 241 influenza-like illness cases in children were recorded in Zhoushan, Zhejiang from week 40, 2016 to week 13, 2020, including 7 184 positive cases. ILI% per week, and the positive rate of influenza virus detection in children per week altered with the season per year, and week per month. Obtained from the MEM model, the pre-epidemic threshold and the post-epidemic threshold of the 2019-2020 influenza season were 22.62% and 28.37% , respectively. The medium, high and extremely high epidemic intensity thresholds were 42.00%, 81.08% and 95.77%.

  Conclusion  The epidemic of influenza in children in Zhoushan, Zhejiang was mainly prevalent in winter and spring, with the mixture of influenza A and B. In addition, MEM model presents a good fitting effect.