The impact of free vaccination policies under the Korean Influenza National Immunization Program: Trends in influenza vaccination rates in South Korea from 2010 to 2019

Background Annual vaccination for influenza is recommended for high-risk populations for its high morbidity and mortality. South Korea provides free influenza vaccination to some target groups under the National Immunization Program (NIP), and discrepantly high vaccination rates are observed in such populations. In this study, we analyzed the trends in influenza vaccination rates and evaluated the impact of the recent expansion of financial coverage to children <= 12 years and pregnant women. Methods We conducted a cross-sectional study with nationwide survey data from Korea National Health and Nutrition Examination Survey (KNHANES). From 2010 to 2019, we evaluated the trends in influenza vaccination rates of the following four target groups: children <= 12 years, adults >= 65 years, pregnant women, and people with chronic diseases. Results In total, 80,861 individuals were analyzed. From 2017 to 2019, the vaccination coverage of children <= 12 years increased from 66.2% to 83.1%; pregnant women from 44.1% to 68.5% (comparing the mean of 2010-2017 and 2018-2019, P <0.001 for both). The elderly >= 65 years showed the highest rates (85.8% in 2019), while people with chronic diseases marked the lowest (41.9% in 2019). People with liver diseases showed the lowest vaccination rate of 27.8%, while that of other common diseases ranged between 31.7-44.1%. Conclusion The discrepancy between target groups corresponds to their financial coverage under NIP. The recent expansion of financial aids to children <= 12 years and pregnant women was followed by significant increases in vaccination rates in both groups. We suggest that free vaccination policy is one of the most effective strategies to enhance vaccination coverage, and we call for its expansion to other under-vaccinated target groups, especially people with chronic diseases.