Associate Professor College of Pharmacy, Chung-Ang University, Korea, Republic of Korea
Background: Influenza is a cause substantial hospitalization due to influenza infection and complications with cardiopulmonary disease. Annual influenza vaccination has been known to be the most effective strategy for protection against influenza and the prevention of influenza-related complications. In Korea, older adults aged 65 years or older are provided influenza vaccination as a national immunization program. The effectiveness of annual influenza vaccination using a nationwide immunization registry and claims database has not been reported in Korea.
Objectives: To evaluate the effectiveness of influenza vaccine in older adults in Korea from 2015/16 to 2018/19 season.
Methods: A nationwide population-based cohort study was conducted using a linked database between the national immunization registry database from Korea disease control and prevention agency and the nationwide healthcare claims database from 2015/16 through 2018/19 season. During the four consecutive seasons, patients were defined as all community-dwelling adults aged 65 years or older at the beginning of each season. Vaccine effectiveness (VE) estimates with a 95% confidence interval (CI) were calculated by using the reduction in the incidence of influenza-like illness (ILI) in the vaccinated group compared to the unvaccinated group. ILI was defined as hospitalizations or emergency department visits due to seasonal influenza (ICD-10: J09, J10, and J11).
Results: During the study period, the population eligible for nationwide influenza VE analyses were 4,663,375 in 2015/2016 season, 4,829,089 in 2016/2017 season, 5,145,125 in 2017/2018 season, and 4,910,011 in 2018/2019 season. The influenza vaccination rate was 85.9%, 86.9%, 87.1%, and 87.8% in the four consecutive seasons, respectively. The overall VE against seasonal influenza was 23.6% (95% CI, 19.0%–28.2%) in the 2015/2016 season, 18.9% (95% CI, 15.1%–22.7%) in the 2016/2017 season, 14.9% (95% CI, 12.4%–17.3%) in the 2017/2018 season, 28.6% (95% CI, 25.0%–32.2%) in the 2018/2019 season.
Conclusions: Influenza vaccination rates have steadily increased from 2015/16 to 2018/19 in Korea. The seasonal influenza vaccine showed effectiveness in preventing influenza-associated illnesses during the four consecutive seasons in Korea.
