Background: Covid-19 has caused significant global mortality . Multiple vaccines have demonstrated efficacy in clinical trials though real-world effectiveness of vaccines against Covid-19 mortality in clinically and demographically diverse populations has not yet been reported .
Methods: We used a retrospective cohort assembled from a cross-institution comprehensive data repository . Established patients of the health care system were categorized as not immunized, partially immunized, or fully immunized against SARS-CoV-2 with an mRNA vaccine through April 4 , 2021 . Outcomes were Covid-19 related hospitalization and death .
Results: Of the 91,134 established patients , 70.2% were not immunized , 4.5% were partially immunized and 25.4% were fully immunized . Among the fully immunized 0.7% had a Covid-19 hospitalization, whereas 3.4% among the partially immunized and 2.7% non-immunized individuals were hospitalized with Covid-19 . Of the 225 deaths among Covid-19 hospitalizations , 219 (97.3 %) were in the not immunized , 5 (2.2 %) in the partially immunized, and 1 (0.0041 %) in the fully immunized group . mRNA vaccines were 96% (95% CI : 95 - 99) effective at preventing Covid-19 related hospitalization and 98.7% (95% CI : 91.0 - 99.8) effective at preventing Covid-19 related death when participants were fully vaccinated . Partial vaccination was 77% (95% CI : 71 - 82) effective at preventing hospitalization and 64.2% (95% CI : 13.0 - 85.2) effective at preventing death . Vaccine effectiveness at preventing hospitalization was conserved across subgroups of age, race, ethnicity, Area Deprivation Index, and Charlson Comorbidity Index .
Conclusions: In a large, diverse cohort in the United States, full immunization with mRNA vaccines was highly effective in the real-world scenario at preventing Covid-19 related hospitalization and death.