Temporal patterns and risk factors of recurrent influenza infection: a population-based epidemiological study

The continuous mutation and seasonal dominance of influenza viruses subject people to the danger of repeated infection. Nonetheless, comprehensive studies on the temporal patterns and risk differentials of influenza reinfection at the population level are few.

The study analyzed surveillance data from 2,834,112 influenza cases in Sichuan Province from 2023 to 2025. Follow-up continued until week 156 after infection. The Kaplan-Meier method measured cumulative reinfection incidence, whereas the Andersen-Gill count process model examined different factors on reinfection risk. Reinfection temporal distribution by age group was characterized using kernel density estimation.

The Kaplan-Meier curves showed a progressive increase in reinfection risk every 52 weeks, reaching 5.5% at 156 weeks. The log-rank test showed a substantial correlation between gender, age, occupation, residence, and case classification and reinfection likelihood (P < 0.001). A multivariate Andersen-Gill analysis showed that people aged 0-5 and 6-19 had a considerably higher reinfection risk than those aged 20-45 and ≥ 61 years. Kernel density analysis showed that reinfection occurred between 9 and 12 months after the original infection, with children having the highest prevalence and adults having a bimodal distribution.

Influenza reinfection risk varies annually, notably in children, adolescents, and high-exposure occupational groups. Significant disparities in risk among groups suggest that age and sociodemographic variables affect influenza immune protection and exposure patterns. Epidemiological data from these studies can improve stratified immunization programs and focused prevention efforts for key groups.