Effectiveness of mRNA COVID-19 Vaccines among Employees in an American Healthcare System

Shrestha, N. K.; Nowacki, A. S.; Burke, P. C.; Terpeluk, P.; Gordon, S. M.

Background. The mRNA SARS-CoV-2 vaccines have shown great promise in clinical trials. The purpose of this study was to evaluate the effectiveness of these vaccines under real-world conditions in the USA. Methods. Employees of the Cleveland Clinic Health System, previously not infected with SARS-CoV-2, and working in Ohio on Dec 16, 2020, the day COVID-19 vaccination began, were included. The cumulative incidence of SARS-CoV-2 infection, over the next 5 months, was compared among those who received the vaccine and those who did not, by modeling vaccination as a time-dependent covariate in Cox proportional hazards regression analyses adjusted for the slope of the epidemic curve as a continuous time-dependent covariate. Results. Of the 46866 included employees, 28223 (60%) were vaccinated by the end of the study period. The cumulative incidence of SARS-CoV-2 infection was much higher among those not vaccinated than those vaccinated. Only 15 (0.7%) of the 2154 SARS-CoV-2 infections during the study occurred among those vaccinated. After adjusting for the slope of the epidemic curve, age, and job type, vaccination was associated with a significantly reduced risk of SARS-CoV-2 infection (HR 0.03, 95% C.I. 0.02 - 0.06, p < 0.001), corresponding to a vaccine effectiveness rate of 97.1% (95% CI 94.3 - 98.5). Vaccine effectiveness was 89.2% at 7 days and 95.0% at 14 days after the first vaccine dose. Conclusions. The mRNA SARS-CoV-2 vaccines are over 97% protective against COVID-19 in the working-age population, with substantial protection possibly apparent within a few days of the first dose.