Risk of SARS-CoV-2 reinfection 18 months after primary infection: population-level observational study.

ABSTRACT

Current data suggest that SARS-CoV-2 reinfections are rare, but uncertainties remain on the duration of the natural immunity, its protection against Omicron variant, finally the impact of vaccination to reduce reinfection rates. In this retrospective cohort analysis of the entire population of an Italian Region, we followed 1,293,941 subjects from the beginning of the pandemic to the current scenario of Omicron predominance (up to mid-January 2022). After an average of 334 days, we recorded 260 reinfections among 84,907 previously infected subjects (overall rate 0.31%), two hospitalizations (2.4 x100,000), and one death. Importantly, the incidence of reinfection did not vary substantially over time after 18-22 months from the primary infection, the reinfection rate was still 0.32%, suggesting that protection conferred by natural immunity may last beyond 12 months. The risk of reinfection was significantly higher among the unvaccinated subjects, and during the Omicron wave.