The study aims to evaluate the effectiveness of mRNA Covid-19 vaccine among U.S Health Care Personnel. The effectiveness in preventing symptomatic Covid-19 was estimated after the first and second dose of BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccines. 

The results show that both vaccines were highly effective among the healthcare workers, including those at risk for severe Covid-19. To be more specific, the effectiveness of partial vaccination was approximately 78% with BNT162b2 vaccine and 89% with mRNA-1273 vaccine, which was higher than the estimates from the respective phase 3 trials. Whereas, the effectiveness of complete vaccination with BNT162b2 vaccine was 89% and with mRNA-1273 vaccine was 96%, which are consistent with efficacy results from phase 3 trials.  

The effectiveness estimated during weeks 9 through 14 were lower than the maximum effectiveness during weeks 3 and 4, but more research should be done to make a conclusion about long-term effectiveness.