Objectives . To evaluate the effect of mRNA SARS-Cov-2 vaccination on the incidence of new SARS-CoV-2 infections in health care workers (HCW). Methods . The evolution of the incident rate of SARS-CoV-2 infection in a cohort of 2590 HCW after a mRNA SARS-CoV-2 vaccination, as compared to the rate in the community (n=170513). SARS-CoV-2 infections were microbiologically confirmed by an antigen, a CRP positive test, or both . Results . A total of 1820 HCW (70,3% of total) received the first dose of the vaccine between January 10-16 , 2021), and 296 (11,4 %) the following week . All of them completed vaccination 3 weeks later . New SARS-COV-2 infections in HCW declined by 62% at 2-4 weeks after the first dose of mRNA SARS-CoV-2 vaccination and virtually disappeared after the second dose of the vaccine . Vaccination rate was negligible for this time period in the community (< 5 %). The decline in the incident rate of SARS-COoV-2 new infection in HCW shortly after the administration of the first dose of the vaccine was strikingly higher than the reduction observed in the general population (p <0.001and became even more pronounced after the second dose of the vaccine (p <0.001). Conclusions . mRNA SARS-CoV-2 vaccination is associated with a dramatic decline in new SARS-CoV-2 infection among HCW, even before the administration of the second dose of the vaccine.