Impact of Vaccination, Prior Infection, and Therapy on Delta and Omicron Variants

We studied 249,070 patients who were tested for SARS-CoV-2 in the Cleveland Clinic Health System between October 1, 2021 and January 31, 2022. We found that vaccination, especially with recent boosting, was more effective than prior infection and monoclonal antibody therapy against both the delta and omicron variants. Vaccination and prior infection were much less effective against infection with the omicron variant than with the delta variant, but the opposite was true of death after infection. Boosting greatly increased the effectiveness of the two mRNA vaccines against both infection and death, although its effects waned markedly after 6 months. In addition, monoclonal antibody therapy was notably less effective at preventing death from the omicron variant than from the delta variant. Finally, the relatively low mortality of the omicron variant was due to both the reduced lethality of this variant and the increased population immunity acquired from booster vaccination and previous infection.