Risk of COVID-19 reinfection and vaccine breakthrough infection, Madera County, California, 2021.

COVID-19 vaccine efficacy has been evaluated in controlled clinical trials and serves as a benchmark for evaluating the protection acquired from prior COVID-19 infection ("natural immunity"). A cohort of persons with a prior COVID-19 infection was matched to a cohort of COVID-19 vaccinated persons, and the risk of reinfection post-COVID-19 infection was compared to the risk of a COVID-19 infection postvaccination. The hazard ratio (HR) for risk of reinfection from day 90 to 300 after initial COVID-19 infection vs vaccine breakthrough infection was 0.48, (95% confidence interval [CI], 0.31-0.73). Thus from 90 to 300 days after COVID-19 infection, the post-COVID-19 infection cohort had a lower risk of COVID-19 infection compared with those fully vaccinated. The risk of death associated with the initial COVID-19 infection requisite for acquiring post-COVID-19 immunity was also assessed. The hazard ratio for deaths from all causes among those acquiring immunity via COVID-19 infection vs vaccination was 14.9 (95% CI, 7.27-30.4). Thus, while post-COVID-19 immunity was on a level comparable to that of vaccination, there was a 15-fold higher mortality resulting from achieving "natural immunity" vs acquiring vaccine-provided immunity.