E-Mail The Clalit Research Institute, in collaboration with researchers from Harvard University, analyzed one of the world's largest integrated health record databases to examine the effectiveness of the Pfizer vaccine against COVID-19. The study provides the first large-scale peer-reviewed evaluation of the effectiveness of a COVID-19 vaccine in a nationwide mass-vaccination setting. The study was conducted in Israel, which currently leads the world in COVID-19 vaccination rates. The results of this study validate and complement the previously reported findings of the Pfizer/BioNTech Phase-III randomized clinical trial, which focused on symptomatic infections, and which, with 21,720 vaccinated individuals, could not precisely assess vaccine effectiveness against severe disease in the fully vaccinated. The present study's large size allows a more detailed assessment of the vaccine's effectiveness in preventing a wider range of outcomes, across different time periods and population sub-groups.