Researchers studied the effect of dose one of mRNA-12273 vaccine in preventing SARS-CoV-2 infection in healthcare workers (HCWs) in the VA Boston Healthcare System. Using a retrospective survival analysis, researchers calculated a hazard ratio of infection in vaccinated versus unvaccinated individuals. Starting on Dec 22, 2020, the 42-day study enrolled a total of 4,028 clinical and non-clinical HCWs. Participants received one dose of the vaccine and a second dose after 28 days and were tested using RT-PCR. The mean age of participants was 48.1 years, with 2,437 women, 2,682 white, and 810 black individuals enrolled. A total of 3,367 HCWs received the vaccine and infection was reported in 107 people, including 39 vaccinated and 68 unvaccinated HCWs. The magnitude of positive tests did not differ significantly based on age, race, gender, number of nurses, and clinical presentation. The vaccine was 50.4% effective for the overall 42-day period, with 77.5% effectiveness days 8-42 and 95% days 15-42. These results show a correlation between the vaccine and protection from SARS-CoV-2 infection after 8 days of one dose, and after day 14, risk of infection reduced by 95% showing potential to reduce the number of infections in HCWs quickly. Limitations include low generalizability due to a small sample size and lack of significant p-values.