Second Booster Vaccine and Covid-19 Mortality in Adults 60 to 100 Years Old

Abstract The rapid emergence of the B.1.1.529 (omicron) variant of severe acute respiratory syndrome coronavirus-2 led to a global resurgence of coronavirus disease 2019 (Covid-19). Israeli authorities approved a 4th Covid-19 vaccine dose (second-booster) for individuals aged 60 and above who received a first booster dose four or more months earlier. Evidence regarding the effectiveness of a second-booster dose in reducing mortality due to Covid-19 is warranted. This retrospective cohort study included all members of Clalit Health Services, aged 60 to 100, eligible for the second-booster. Mortality due to Covid-19 among participants who received the second-booster was compared with participants who received one booster dose. A Cox proportional-hazards regression model with time-dependent covariates was used to estimate the association between the second-booster and death due to Covid-19 while adjusting for demographic factors and coexisting illnesses. A total of 563,465 participants met the eligibility criteria. Of those, 328,597 (58%) received a second-booster dose during the 40-day study period. Death due to Covid-19 occurred in 92 second-booster recipients and in 232 participants who received one booster dose (adjusted hazard ratio 0.22; 95% confidence interval 0.17 to 0.28). This study demonstrates a substantial reduction in Covid-19 mortality by the second-booster in eligible subjects.