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.