LAUSR

Dear Editor, Incident severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections are occurring among vaccinated individuals [1]. While it is known that prior coronavirus disease 2019 (COVID19) is protective against incident SARSCoV-2 infection [2, 3], it is not clear how much extra protection is conferred to individuals who both are vaccinated and had prior COVID-19 when compared with people who are only vaccinated. We aimed to measure the difference in incident infection among a cohort of vaccinated employees who were routinely tested for COVID-19. In March 2020, Curative, a SARSCoV-2 testing company, began routinely screening its workforce with a Food and Drug Administration–authorized SARS-CoV-2 polymerase chain reaction (PCR)–based test [4]. The workforce was screened daily. A standardized employee testing database was implemented on May 8, 2020. On December 15, 2020, SARS-CoV-2 vaccinations became available, and dates of vaccination were recorded. Vaccinations were required for employees. Routine screening has continued through December 2021. The SARS-CoV-2-naïve, vaccinated group was defined as any employee without previous SARS-CoV-2 infection. The previously infected, vaccinated group was defined as any employee with documented previous SARS-CoV-2 infection (at least 2 positive sequential PCR tests). Individuals were added to the cohort when they completed a vaccination form and were followed until December 20, 2021. Person-days were measured from the date reported vaccination to last test date up to December 20, 2021. A period of at least 3 weeks was given between infections before a new incident infection would be counted. The incidence rate ratio (IRR), the ratio of confirmed COVID-19 cases per 100 person-years of follow-up, with 95% confidence intervals, was calculated. Analyses were performed on StataSE (StataCorp, College Station, TX, USA).