Unintended beneficial effects of COVID-19 on influenza-associated emergency department use in Korea

BACKGROUND: Non-pharmaceutical interventions, including hand hygiene, wearing masks, and cough etiquette, and public health measures such as social distancing, used to prevent the spread of coronavirus disease 2019 (COVID-19), could reduce the incidence rate of respiratory viral infections such as influenza. We evaluated the effect of COVID-19 on the incidence of influenza in Korea. METHODS: This retrospective study included all patients who visited five urban emergency departments (EDs) during the influenza epidemic seasons of 2017–18, 2018–19, and 2019–20. Influenza was defined as ICD-10 codes J09, J10, and J11, determined from ED discharge records. The weekly incidence rates of influenza per 1000 ED visits during the 2019–20 season, when COVID-19 became a pandemic, were compared with those of 2017–18 and 2018–19. The actual incidence rate of the 2019–20 season was compared with the predicted value using a generalized estimation equation model based on 2017–18 and 2018–19 data. RESULTS: The weekly influenza incidence rate decreased from 101.6 to 56.6 between week 4 and week 5 in 2020 when the first COVID-19 patient was diagnosed and public health measures were implemented. The weekly incidence rate during week 10 and week 22 of the 2019–20 season decreased most steeply compared to 2017–18 and 2018–19. The actual influenza incidence rate observed in the 2019–20 season was lower than the rate predicted in the 2017–18 and 2018–19 seasons starting from week 7 when a COVID-19 outbreak occurred in Korea. CONCLUSIONS: The implementation of non-pharmaceutical interventions and public health measures for the COVID-19 epidemic effectively reduced the transmission of influenza and associated ED use in Korea. Implementing appropriate public health measures could reduce outbreaks and lessen the burden of influenza during future influenza epidemics.