Outcomes of vaccinated versus unvaccinated COVID-19 patients in Israel during the Omicron and Delta waves-A retrospective cohort study.

INTRODUCTION:Initiation of global vaccination significantly reduced the morbidity and mortality of COVID-19. During the Omicron wave, approximately 70% of the Israeli adult population was fully vaccinated, but the efficacy of the vaccine was questioned. METHODS:We conducted a retrospective cohort study of all adult patients admitted to the COVID-19 departments in Rabin Medical Center, during the Delta wave and the Omicron wave. Patients were matched in the 2 waves using the inverse probability of treatment weighting (IPTW) method and risk for mechanical ventilation and 30-day all-cause mortality was assessed. RESULTS:Vaccination had a significant effect on 30-day mortality in the Delta and Omicron waves with adjusted OR of 0.35 (0.17-0.70) and 0.5 (0.27-0.95) respectively. Nonetheless, the rate of mechanical ventilation was similar between the groups with OR of 0.75 (0.52-1.09) and 0.64 (0.40-1.01). Vaccination status did not change the length of admission in both waves. CONCLUSION:We observed a decreased risk for 30-day mortality among vaccinated patients during the Delta and Omicron waves in Israel. This association, even though consistent, was of a lesser magnitude during the Omicron wave.