Incidence of SARS-CoV-2 infection among previously infected or vaccinated employees

Introduction: We aimed to determine the incidence of SARS-CoV-2 infection among individuals with a previous SARS-CoV-2 infection versus vaccinated individuals.

Methods: In March 2020, a SARS-CoV-2 testing company began routinely screening its workforce for SARS-CoV-2 with a PCR test. On December 15, 2020, vaccination with either the BNT162b2 or mRNA-1273 vaccines became available. Routine screening has continued through July 2021. We compared the incidence of SARS-CoV-2 infection between people who were SARS-CoV-2 naïve and unvaccinated, people with prior COVID-19 without vaccination, and people vaccinated without prior COVID-19. Incidence in 100 person-years with 95% confidence intervals (95% CIs) was calculated with the Poisson Exact equation. The incidence rate ratio (IRR), the ratio of confirmed COVID-19 cases per 100 person-years of follow-up with 95% CIs, was used as a measure of association between groups. Analyses were performed on StataSE.

Results: The median age of employees was 29.0 years (interquartile range: 23.6, 39.9). During the observation period, 258 SARS-CoV-2 incident infections were identified. The naïve, unvaccinated group had a SARS-CoV-2 incidence of 25.9 per 100 person-years (95% CI: 22.8-29.3). The previously infected, unvaccinated group had an incidence of 0 per 100 person-years (95% CI: 0-5.0). The vaccinated group had an incidence of 1.6 per 100 person-years (95% CI: 0.04-4.2).

Conclusion: We found a strong association between prior SARS-CoV-2 infection and/or vaccination for SARS-CoV-2 with either the BNT162b2 or mRNA-1273 vaccines and the reduced incidence of SARS-CoV-2 infection when compared with those naïve and/or unvaccinated to SARS-CoV-2.