Epidemiological characteristics and incidence trend of influenza like illness in Hangzhou, Zhejiang, 2014−2023

Objective To understand the epidemiological characteristics and incidence trend of influenza-like illness (ILI) in Hangzhou, Zhejiang province, from 2014 to 2023, and provide evidence for the prevention and control of influenza in the future.

Methods The local incidence data of ILI from 2014 to 2023 were collected from sentinel hospitals in Hangzhou, and descriptive statistical methods were used to analyze the epidemiological characteristics of ILI. A segmented-point linear Joinpoint regression model with the year as independent variable and the proportion of ILI cases in total outpatient visits as dependent variable was constructed to explore the incidence trend of ILI.

Results From 2014 to 2023, a total of 1 788 669 ILI cases were reported cumulatively by 15 influenza sentinel hospitals in Hangzhou, and the proportion of ILI cases was 2.85% in total outpatient visits. This proportion was highest in age group under 15 years（53.11%）, and the annual proportion in urban area（3.38%） was higher than that in suburban area（2.26%） (P<0.001). During this period, a total of 23 471 throat swabs were collected from ILI cases in two sentinel hospitals at national level, in which 4 346 were positive for influenza viruses, and the etiological positive rate was 18.52%. The main types were influenza A (H3N2) virus, influenza B virus Victoria lineage, and influenza A (H1N1) pdm09 virus, and no influenza B virus Yamagata lineage was detected after 2019. Influenza mainly occurred in winter and spring, with sub-peaks in summer in some years. There was a strong correlation between the ILI case proportion and the positive rate, and the time cross-correlation analysis indicated that the ILI case proportion predicted the incidence peak 1 week earlier compared with the positive rate. The Joinpoint regression analysis showed that overall ILI case proportion showed an upward trend without obvious change during this period, the average annual percent change (AAPC)=8.48% (P=0.007). Both urban and suburban ILI case proportions showed upward trend, but the changes were unbalanced (P＜0.001), with the urban change rate (AAPC=3.58%, P=0.268) being less than that of suburban areas (AAPC=19.07%, P=0.001), and no obvious changes were observed in either urban or suburban areas.

Conclusion ILI mainly occurred in winter and spring in Hangzhou, with sub-peaks in summer in some years. The intensity of influenza activity was stronger in Hangzhou than in northern regions but slightly lower than in southern regions. Children under 15 years old are the key population in influenza prevention and control. The ILI case proportion can predict the incidence peak one week earlier. Over the past decade, the ILI case proportion showed a year-by-year increase, with rapid increase in suburban area to the incidence level in urban areas, to which close attention needs to be paid.