Incidence of and risk factors for suspected COVID-19 reinfection in Kyoto City: a population-based epidemiological study

To determine the clinical characteristics of and risk factors for suspected reinfection with coronavirus 2019 (COVID-19). This was a retrospective cohort study using population-based notification records of residents in Kyoto City (1.4 M) with laboratory-confirmed COVID-19 infection between 1 March 2020 and 15 April 2022. Reinfection was defined by two or more positive COVID-19 test results ≧ 90 days apart. Demographic characteristics, the route and timing of infection and history of vaccination were analysed to identify risk factors for reinfection. Among the cohort of 107,475 patients, reinfection was identified in 0.66% (n = 709). The age group with the highest reinfection rate was 18–39 years (1.06%), followed by 40–59 years (0.58%). Compared to the medical and nursing professionals, individuals who worked in the construction and manufacturing industry (odds ratio [OR]: 2.86; 95% confidence interval [CI]: 1.66–4.92) and hospitality industry (OR: 2.05; 95% CI: 1.28–.31) were more likely to be reinfected. Symptomatic cases at initial infection, receiving more than 2 doses of vaccination and risk factors for severe infection at initial infection were protective factors against reinfection. Of the reinfected individuals, the reinfection route was unknown in 65%. Reinfection with COVID-19 is uncommon, with suspected reinfections more likely in adults, those with high exposure and unvaccinated individuals; the reinfection route was unknown in the majority of cases. This study confirmed the need to continue with self-protection efforts and to implement vaccination programs in high-risk populations. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10096-023-04625-6.