Age-Period-Cohort Analysis of Influenza in Koreans: the National Health Insurance Research Database, 2009–2018

BACKGROUND: This study aimed to identify the incidence rate of episodes diagnosed with influenza and the effects of age-period-cohort (APC) in Koreans. METHODS: The 2009–2018 National Health Insurance Research Database was used for analysis. All time-related claims connected relatively short window period in 100 days. The case definition was defined by all codes diagnosed with J09, J10, and J11. Calculation of the incidence rate and APC analysis adjusted income levels by insurance type, metropolitan city was performed to identify the characteristics of episodes diagnosed with influenza. RESULTS: Incidence rate by age and cohort gradually increased since 2014. The incidence rate of males aged 0–4 years was 171.02 and that of females was 173.31 in 2015–2016 season. In males, 29.19 in 1963 cohort and 243.79 in 2013 cohort were confirmed as high incidence rates in 2017–2018 season. In the females, a high incidence was confirmed in 1953–1967 cohort and 1978–1987 cohort, and the incidence was 251.38 in 2013–2017 cohort. APC effects showed a high relative risk in the infants, the pandemic influenza season in 2010 (1/7/2009 to 30/6/2010) and the adults of 1978–1987 cohort. CONCLUSION: Since 2014, influenza outbreaks have been increasing every year. The start year of free vaccination decreased the incidence in infants and adults over 65 years of age but the incidence increased from the following year. Because influenza can be primarily prevented by vaccination, reinforcement of vaccination in infants may reduce the disease burden in their parents, and also the risk of infection caused by family transmission. A new vaccination strategy is needed to reduce the incidence and burden of diseases caused by influenza infection.