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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. Hospitalizations and death due to Covid-19 among participants who received the bivalent vaccine were compared with those who did not. A Cox proportional-hazards regression model with time-dependent covariates was used to estimate the association between the bivalent vaccine and Covid-19 outcomes while adjusting for demographic factors and coexisting illnesses.
Findings: A total of 622,701 participants met the eligibility criteria. Of those, 85,314 (14%) received a bivalent-booster during the 70-day study period. Hospitalization due to Covid-19 occurred in 6 bivalent recipients and 297 participants who did not, adjusted hazard ratio (HR): 0.19 (95% CI, 0.08-0.43). Death due to Covid-19 occurred in 1 bivalent recipient and 73 participants who did not, adjusted HR 0.14: (95% CI, 0.02-1.04).
Interpretation: Participants who received the bivalent vaccine had lower hospitalization and mortality rates due to Covid-19 than non-recipients up to 70 days after vaccination.
Funding Information: None.
Preprint available at the Lancet:
