National trends in influenza vaccination coverage rates in South Korea between 2007-2020, including the COVID-19 pandemic: a longitudinal nationwide serial study

Objective: The beneficial relationship of influenza vaccination rate on the COVID-19 pandemic is inconclusive and inconsistent. Thus, we aimed to investigate long-term trend changes in influenza vaccination rate among general population of South Korea before COVID-19 pandemic and during early/mid-pandemic period. Methods: We analyzed nationwide representative serial study from the Korea National Health and Nutrition Examination Survey (KNHANES). We analyzed data from 2007 to 2020 from KNHANES to calculate the influenza vaccination rates. Influenza vaccination status was obtained from health interviews and questionnaire based on the question of whether the influenza vaccine was vaccinated in the past 12 months. Results: Among 78,067 participants, the vaccination coverage increased from 2007 to 2020 (27.6% in 2007 to 2008 and 42.2% in 2020). The slope for the vaccination coverage increased between 2007 to 2019 (pre-pandemic, β= 0.310, 95% CI, 0.267 to 0.352), and the slope increased less than expected between 2018 to 2020 (entering the pandemic, β = 0.036, 95% CI, 0.010 to 0.062; β diff = -0.274, 95% CI, -0.323 to -0.224). However, the participants with ≥ 65 years old and those with medical condition (diabetes, tuberculosis, asthma, angina, myocardial infarction, cancer, chronic kidney disease, hypertension, and hyperlipidemia), the trends of the vaccination coverage increased from 2007 to 2019, whereas the coverage were decreased in 2020 (≥65 years old: 72.9%, 85.5%, and 81.4%; medical condition: 43.3%, 56.3%, and 55.7%, respectively). Conclusions: In this study using nationwide serial study, the influenza vaccination coverage increased from 2007 to 2020, and the slope increased less than expected in 2020 during the COVID-19 pandemic. On the other hand, the vaccination coverage decreased during the COVID-19 pandemic in the participants with ≥65 years old and those with medical condition. We suggest that influenza vaccination should be encouraged to the elderly and chronic disease patients to prevent twin infection during the COVID-19 pandemic.