Influenza-like illness and etiological surveillance in Zhenjiang, Jiangsu, 2013–2020

  Objective  To understand the epidemiological and etiological characteristics of influenza-like illness (ILI) in Zhenjiang, Jiangsu province, during 2013–2020, and provide scientific basis for the prevention and control of influenza.

  Methods  ILI surveillance system consisting of influenza surveillance sentinel hospitals and influenza laboratory network in Zhenjiang was used to collect the surveillance data of ILI cases and pathogens in Zhenjiang from 2013 to 2020 for a statistical analysis.

  Results  A total of 207 252 ILI cases were reported by sentinel hospitals in Zhenjiang during 2013–2020, and the average ILI% during 2013–2020 was 3.50%, showing an increase trend (trend χ²=30554.00, P<0.001). The ILI cases in age groups 0–4, 5–14, 15–24, 25–59 and ≥60 years accounted for 39.91%, 27.53%, 7.40%, 17.93%, 7.23% respectively. There were two peaks of ILI% in the same influenza surveillance year, December-February and June-September. A total of 25 318 ILI samples were tested in Zhenjiang during 2013–2020, in which 2 780 were influenza virus positive (10.98%). The detection rate of influenza A virus (7.08%) was higher than influenza B virus (3.90%). Influenza A (H1N1) virus was mainly detected during January-March, influenza A (H3N2) was mainly detected during July-December, influenza B virus Victoria lineage was mainly detected during April-June, and influenza B virus Yamagata lineage was mainly detected during January-March. There were two types of influenza virus epidemics, one was caused by single influenza virus, and another one was caused by multi influenza viruses. A total of 39 ILI outbreaks were reported during 2013–2019 in Zhenjiang, in which 37 occurred in schools and child care settings, no ILI outbreak was reported in 2020. During COVID-19 epidemic period in 2020, the ILI% (6.82%) was 125.08% higher than the average during 2013–2019 (3.03%), and the increase was highest in age group ≥60 years (176.61%). The positive rate of influenza virus was 52.21% in January, 11.20% in February, 0.37% in March, 0 in April-December in 2020, the positive rate in 2020 (5.37%) was 54.41% lower than the average during 2013–2019 (11.79%).

  Conclusion  It is necessary to strengthen the prevention and control of influenza according to the epidemiological characteristics of ILI and influenza virus in Zhenjiang. The prevention and control measures for COVID-19 can be used for reference.