Background
The protective efficacy of prior COVID-19 with or without vaccination remains unknown. This study sought to understand if two or more mRNA vaccine doses provide additional protection in patients with prior infection, or if infection alone provides comparable protection.
Methods
We conducted a retrospective cohort study of the risk of COVID-19 from December 16, 2020, through March 15, 2022, among vaccinated and unvaccinated patients of all ages with and without prior infection. A Simon-Makuch hazard plot illustrated the incidence of COVID-19 between groups. Multi-variable Cox proportional hazards regression was used to examine the association of demographics, prior infection, and vaccination status with new infection.
Results
Among 101,941 individuals with at least one COVID-19 PCR test prior to 3/15/2022, 72,361 (71.0%) received mRNA vaccination and 5,957 (5.8%) were previously infected. The cumulative incidence of COVID-19 was substantially higher throughout the study period for those previously uninfected and unvaccinated, and lowest for those previously infected and vaccinated. After accounting for age, sex, and the interaction between vaccination and prior infection, a reduction in reinfection risk was noted during the Omicron and pre-Omicron phases of 26% (95% CI 8%, 41%, p= .0065) to 36% (95% CI 10%, 54%, p=0.0108), respectively, among previously infected and vaccinated individuals, compared to previously infected subjects without vaccination.
Conclusion
Vaccination was associated with lower risk of COVID-19, including in those with prior infection. Vaccination should be encouraged for all including those with prior infection, especially as new variants emerge, and variant-specific booster vaccines become available.