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Qatar introduced COVID-19 bivalent vaccination for persons ≥12 years old using the 50-μg mRNA-1273.214 vaccine combining SARS-CoV-2 ancestral and omicron BA.1 strains. We estimated effectiveness of this bivalent vaccine against SARS-CoV-2 infection using a matched, retrospective, cohort study. Matched cohorts included 11,482 persons in the bivalent cohort and 56,806 persons in the no-recent-vaccination cohort. During follow-up, 65 infections were recorded in the bivalent cohort and 406 were recorded in the no-recent-vaccination cohort. None progressed to severe, critical, or fatal COVID-19. Cumulative incidence of infection was 0.80% (95% CI: 0.61-1.07%) in the bivalent cohort and 1.00% (95% CI: 0.89-1.11%) in the no-recent-vaccination cohort, 150 days after the start of follow-up. Incidence during follow-up was dominated by omicron XBB* subvariants including XBB, XBB.1, XBB.1.5, XBB.1.9.1, XBB.1.9.2, XBB.1.16, and XBB.2.3. The adjusted hazard ratio comparing incidence of infection in the bivalent cohort to that in the no-recent-vaccination cohort was 0.75 (95% CI: 0.57-0.97). Bivalent vaccine effectiveness against infection was 25.2% (95% CI: 2.6-42.6%). Effectiveness was 21.5% (95% CI: -8.2-43.5%) among persons with no prior infection and 33.3% (95% CI: -4.6-57.6%) among persons with prior infection. mRNA-1273.214 reduced incidence of SARS-CoV-2 infection, but the protection was modest at only 25%. The modest protection may have risen because of XBB* immune evasion or immune imprinting effects, or combination of both. |
