Influenza Vaccine Effectiveness among Elementary School Students in Korea during the 2016-2017 Seasons: a Cross-Sectional Survey.

Background: Annual vaccination is the principal way to reduce the mortality and morbidity associated with influenza. In the 2016-2017 influenza seasons, the influenza epidemic appeared to exhibit a different pattern from the previous years. Because of the unusual trend, the incidence of influenza-like patients among school-aged children had increased, causing doubts about the effectiveness of the influenza vaccine. Therefore, this study aimed to evaluate the effectiveness of the influenza vaccine among elementary school students in Korea. Methods: The study was conducted in elementary schools in each province of Korea in cooperation with the Student Health Policy Division of the Ministry of Education. Each Provincial Office of Education of Korea, except for Jeju, randomly selected one to two elementary schools for each District Office of Education. A total of 2,739 elementary school students were enrolled and vaccination and influenza infection status were collected from the subjects' parents through questionnaires, from February 13th to February 21st in 2017. Vaccine effectiveness was defined as calculating the infection rate of influenza among the vaccinated and unvaccinated groups and determining the decreased infection rate of the vaccinated group relative to the unvaccinated group, while adjusting for time of vaccination and infection. Results: Adjusting for the interval between vaccination and infection, vaccine effectiveness of influenza was 17.6% (95% confidence interval (CI], 4.6 degrees/0 to 28.9%), 22.5% (95% CI, 10.3% to 33%), and 28.7% (95% CI, 17.5% to 38.3%) at 2 or more weeks, 3 or more weeks, and 4 or more weeks after vaccination, respectively. Conclusion: In conclusion, considering the time required for adequate immunogenicity, the 2016-2017 seasonal influenza vaccine effectiveness in Korean elementary school students was 17.6%-28.7%, which was less effective than that of previous years.