Background: During Late 2022, the SARS-CoV-2 Omicron BA.5 sublineages accounted for most of the sequenced viral genomes worldwide. Bivalent mRNA vaccines contain an ancestral SARS-CoV-2 strain component plus an updated component of the Omicron BA.4/BA.5. Since September 2022, a single bivalent booster dose has been recommended for adults who have completed a primary vaccination series and are at high risk for severe Covid-19 disease. Evidence regarding the effectiveness of the bivalent vaccine in reducing hospitalizations and death due to Covid-19 is warranted.Methods: This retrospective cohort study included all members of Clalit Health Services, aged ≥65, eligible for a bivalent booster....