Characterization of influenza epidemic in Changzhou City， Jiangsu Province from 2019 to 2023

Objective To analyze the epidemic intensity and characteristics of influenza in Changzhou City， Jiangsu Province from 2019 to 2023， and to provide scientific evidence for the formulation of influenza prevention and control strategies.

Methods The surveillance data of influenza surveillance sentinel hospitals in Changzhou City from April 2019 to March 2023 were collected through the China influenza surveillance information system. Influenza-like illness （ILI） cases before and after the pandemic of COVID-19 in Changzhou City were analyzed， and the differences among qualitative data were compared by using χ^² test.

Results From April 2019 to March 2023 the percentages of ILI cases’ medical visits in the four-influenza surveillance year were 2.57%， 1.84%， 5.38%， and 3.66%， respectively， and the positive detection rates of influenza virus were 25.71%， 0.44%， 22.78%， and 24.32%， respectively. The number of influenza outbreaks was 61， 1， 23， and 128， respectively. ILI cases were mainly among adolescent children aged 5‒14 years. The percentage of ILI cases in 2020‒2021 after the pandemic of COVID-19 was significantly lower than that in 2019‒2020 （χ²=737.342， P<0.001）， and the percentage of ILI cases in 2021‒2023 was higher than that in 2019‒2020. Influenza viruses in 2019‒2021 were dominated by type B Victoria， and influenza A virus was the dominant strain in positive influenza virus detections in 2022‒2023. The number of influenza outbreaks in 2021‒2023 was significantly lower than that in 2019‒2020 （χ²=185.662， P<0.001）.

Conclusion The epidemiological characteristics of influenza are different in different stages of COVID-19 prevention and control during 2019‒2023. In the dynamic clearance phase of COVID-19， ILI case’s medical visits showed low-level fluctuations without obvious seasonal fluctuations， and influenza virus was dominated by type B. In the high-intensity phase of the pandemic， the level of ILI case’s medical visits has reached the peak of the calendar year， and the positive detection of influenza virus was dominated by type A H3N2. In the "Category B B control" phase， the level of ILI case’s medical visits increased after a rapid decline， and the positive detection of influenza virus was dominated by type A H1N1. It is necessary to further strengthen the monitoring of influenza cases， pay close attention to the changes in influenza strains， actively promote influenza vaccination for key population group， and promote health behavior changes for the whole population.