Effectiveness of Booster mRNA Vaccines Against SARS-CoV-2 Infection in an Elderly Population, South Korea, October 2021–January 2022

To the Editor—We read with great interest the recent article by Drawz et al in which the authors explored the potential role of messenger RNA (mRNA) vaccine’s booster doses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–related hospitalizations [1]. In South Korea, a coronavirus disease 2019 (COVID-19) vaccination campaign was initiated starting on 26 February 2021. The primary series vaccines available for persons aged 60+ years were ChAdOx1 nCoV-19 (AstraZeneca) and BNT162b2 mRNA (Pfizer BioNTech) COVID-19 vaccines, whereas booster vaccinations with mRNA vaccines were recommended starting in October 2021. We evaluate the impact of the booster mRNA vaccines against SARS-CoV-2 in preventing infection, severe disease, and death in all persons aged 60+ years in South Korea. We conducted a nationwide retrospective cohort study to estimate the impact of booster vaccination in all persons aged 60+ years who received 2 doses of COVID-19 vaccines at least 5 months...