Long-term retrospective analysis of seasonal influenza epidemic patterns in Southeastern Province of China: based on case reports, syndromic, and pathogenic surveillance.

backgroundSeasonal influenza poses a significant public health burden. In the subtropical monsoon climate region of southern China, its epidemiological patterns and the correlations between different surveillance systems over the past decade are poorly understood. To address these issues, we conducted a long-term retrospective analysis of seasonal influenza epidemic patterns in Jiangxi, southeastern China.

methodsWe synchronously conducted case reports and syndromic and pathogenic surveillance from 2014 to 2023. Descriptive analysis was performed to characterize the epidemic trends. Joinpoint regression analysis was used to assess the temporal changes in the reported incidence of seasonal influenza. Wavelet analysis was conducted to identify the periodicity of seasonal influenza epidemics, and polynomial regression was used to assess the relationships between different surveillance systems.

resultsFrom 2014 to 2023, Jiangxi reported 417,636 seasonal influenza cases (average annual reported incidence: 91/100,000), showing an increasing trend, with a slow increase from 2014 to 2021 (annual percentage change [APC] = 17%, 95% confidence interval [CI] = −17%–32%, p = 0.232) and a subsequent sharp rise from 2021 to 2023 (APC = 154%, 95%CI = 39–304%, p < 0.001). Seasonal trends revealed bimodal epidemic peaks annually during winter and summer months. Influenza A and B exhibited periodicities of 39 and 19 weeks, and 51 and 29 weeks, respectively. The proportion of reported cases in the 6–14 year age group gradually exceeded that in the 15–59 year age group, becoming the dominant population after 2023. Males exhibited higher reported incidence rates than females in both the 0–2 and 60+ year age groups during all the surveillance years. The syndromic surveillance system demonstrated general concordance with case report data. Polynomial regression analysis revealed a J-shaped relationship between the detected positive rate and incidence, and a linear correlation between influenza - like illness (ILI)% and incidence.

conclusionsThe incidence of influenza in Jiangxi Province has risen significantly over the past decade, with dynamic changes in age-specific distribution. Public health interventions for school-age children, such as strengthening vaccination programs, should be enhanced. The three surveillance systems showed regular relationships, and the adjusted ILI% was a reliable indicator for influenza early warning. The generalizability needs further validation across regions and seasons. CLINICAL TRIAL: Not applicable.