Incidence of SARS‐CoV‐2 infection among COVID‐19 vaccinated and unvaccinated healthcare personnel, first responders, and other essential and frontline workers: Eight US locations, January–September 2021

Background We sought to evaluate the impact of changes in estimates of COVID‐19 vaccine effectiveness on the incidence of laboratory‐confirmed infection among frontline workers at high risk for SARS‐CoV‐2. Methods We analyzed data from a prospective frontline worker cohort to estimate the incidence of COVID‐19 by month as well as the association of COVID‐19 vaccination, occupation, demographics, physical distancing, and mask use with infection risk. Participants completed baseline and quarterly surveys, and each week self‐collected mid‐turbinate nasal swabs and reported symptoms. Results Among 1018 unvaccinated and 3531 fully vaccinated workers, the monthly incidence of laboratory‐confirmed SARS‐CoV‐2 infection in January 2021 was 13.9 (95% confidence interval [CI]: 10.4–17.4), declining to 0.5 (95% CI ‐0.4‐1.4) per 1000 person‐weeks in June. By September 2021, when the Delta variant predominated, incidence had once again risen to 13.6 (95% CI 7.8–19.4) per 1000 person‐weeks. In contrast, there was no reportable incidence among fully vaccinated participants at the end of January 2021, and incidence remained low until September 2021 when it rose modestly to 4.1 (95% CI 1.9–3.8) per 1000. Below average facemask use was associated with a higher risk of infection for unvaccinated participants during exposure to persons who may have COVID‐19 and vaccinated participants during hours in the community. Conclusions COVID‐19 vaccination was significantly associated with a lower risk of SARS‐CoV‐2 infection despite Delta variant predominance. Our data demonstrate the added protective benefit of facemask use among both unvaccinated and vaccinated frontline workers.