Introduction: Differences in seasonal pattern and age-related infection risk have been reported between Influenza type A and B, but have not been elaborated at subtype/lineage level.

Methodology: All laboratory-confirmed influenza cases reported in the influenza surveillance system of Jiangsu Province, China from January 2011 to August 2019 were analyzed. Influenza seasonality was characterized using the Seasonal Decomposition method. Binary and multinomial logistic regressions were employed to calculate the odds ratios of influenza subtypes/lineages in relation to age.

Results: A total of 28,772 confirmed influenza cases were included. Among them, a majority (64.1%) were influenza A infections. One annual peak was observed for A (H1N1) pdm and B-Yamagata in winter months, and for B-Victoria in spring months, while biannual peaks were observed for A (H3N2) in winter and summer months. Using all ages as the reference, children younger than 5 years and adults of 25-59 years were more likely to infect with A (H1N1) pdm. Older children aged 5-14 years had significantly higher odds of infection with influenza B of both lineages, while individuals aged 15-24 years had higher odds for A (H3N2) and B-Victoria. The elderly (60 years or older) were prone to be infected with A (H3N2) and B-Yamagata. For a specific age group, their likelihood of getting infected with an influenza subtype or lineage was related to the co-circulating influenza subtype or lineage.

Conclusions: Influenza viruses have divergent seasonal peak times and age-related infection risk at subtype/lineage level.