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To better understand the epidemiology of SARS‐CoV‐2 reinfections, we analyzed national data from South Korean who were followed longitudinally from January 2020 to April 2022. We conducted a nationwide retrospective cohort study to estimate possible SARS‐CoV‐2 reinfection rates in all residents in South Korea, with at least two episodes of laboratory‐confirmed SARS‐CoV‐2 infection by reverse‐transcriptase polymerase chain reaction or rapid antigen test (RAT) performed at least 45 or more days between both episodes, between January 2020 and April 2022. There were 16 130 855 laboratory‐confirmed SARS‐CoV‐2 cases in South Korea, with 55 841 (346.2 per 100 000; or 0.3% of all infections) cases of possible reinfections. The reinfection rate has increased from 6.0 cases per 100 000 during Pre‐Delta period to 128.0 cases per 100 000 and 355.1 cases per 100 000 during Delta and Omicron periods, respectively. Persons with one dose of vaccination had the highest reinfection rate of 642.2 per 100 000, followed by unvaccinated persons (536.2/100 000) and two‐dose vaccinated persons (406.3/100 000). Our finding suggests that the majority of possible reinfections occurred following the emergence of new variants. [ABSTRACT FROM AUTHOR]