Incidence of Severe Acute Respiratory Syndrome Coronavirus-2 infection among previously infected or vaccinated employees

IntroductionThe protective effect of previous infection versus vaccination is poorly studied. Among a clinical laboratory that has been conducting routine workforce screening since the beginning of the pandemic, we aimed to assess the relative risk of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection among individuals who were SARS-CoV-2 naïve, previously infected, or vaccinated.MethodsUsing an electronic laboratory information system, employees were divided into three groups: (1) SARS-CoV-2 naïve and unvaccinated, (2) previous SARS-CoV-2 infection, and (3) vaccinated. Person-days were measured from the date of the employee first test and truncated at the end of the observation period. SARS-CoV-2 infection was defined as two positive SARS-CoV-2 PCR tests in a 30-day period. Individuals with fewer than 14 days of follow up were excluded. Incidence estimates and the 95% confidence intervals were calculated using the Poisson Exact equation. The incidence rate ratio (IRR) was used as a measure of association between groups. Analyses were performed on StataSE (StataCorp, College Station, TX).ResultsWe identified 4313, 254 and 739 employee records for groups 1, 2, and 3, respectively. The median age of employees was 29.0 years (interquartile range: 23.6, 39.9). During the observation period, 254, 0, and 4 infections were identified among groups 1, 2, and 3, respectively. Group 1 had an incidence of 25.9 per 100 person-years (95% CI: 22.8-29.3). Group 2 had an incidence of 0 per 100 person-years (95% CI: 0-5.0). Group 3 had an incidence of 1.6 per 100 person-years (95% CI: 0.04-4.2). The IRR of reinfection among those with previous infection compared to SARS-CoV-2 naïve was 0 (95% CI: 0-0.19). The IRR of those vaccinated compared to SARS-CoV-2 naïve was 0.06 (95% CI: 0.02-0.16). The IRR of those vaccinated compared to prior SARS-CoV-2 was 0 (95% CI: 0-4.98).ConclusionPrevious SARS-CoV-2 infection and vaccination for SARS-CoV-2 were associated with decreased risk for infection or re-infection with SARS-CoV-2 in a routinely screened workforce. The was no difference in the infection incidence between vaccinated individuals and individuals with previous infection. Further research is needed to determine whether our results are consistent with the emergence of new SARS-CoV-2 variants.