Study: Third COVID mRNA vaccine dose needed against Omicron

mRNA COVID-19 vaccines are highly effective in preventing adult hospitalizations from the Alpha, Delta, and Omicron SARS-CoV-2 variants, but three doses are needed to reach the same efficacy against Omicron as two doses offer against the first two strains, suggests an observational test-negative study yesterday in BMJ.A team led by University of Michigan at Ann Arbor and Centers for Disease Control and Prevention (CDC) researchers prospectively estimated the effectiveness of the Pfizer/BioNTech and Moderna vaccines against COVID-19 hospitalization among 5,728 adult COVID-19 patients and 5,962 uninfected controls at 21 US hospitals.Consistent with increasing vaccine uptake in the United States over time, a greater share of patients had received two or three doses of an mRNA vaccine during the Omicron period (52.1%) than in periods in which Alpha (11.1%) and Delta (27.3%) were predominant.To identify the causative strain, the researchers used whole-genome sequencing or the predominant circulating variant at hospital admission (Alpha, Mar 11 to Jul 3, 2021; Delta, Jul 4 to Dec 25, 2021; and Omicron, Dec 26, 2021, to Jan 14, 2022).Whole-genome sequencing was obtained for 45.4% of cases.

Among them, Alpha was identified in 57.5% of cases in the Alpha period, Delta in 96.7% in the Delta period, and Omicron in 76.6% in the Omicron period.3 doses 86% effective against Omicron hospitalizationEstimated vaccine effectiveness (VE) against COVID-19 hospitalization was 85% (95% confidence interval [CI], 82% to 88%) for two doses against Alpha, 85% (95% CI, 83% to 87%) for two doses against Delta, 94% (95% CI, 92% to 95%) for three doses against Delta, 65% (95% CI, 51% to 75%) for two doses against Omicron, and 86% (95% CI, 77% to 91%)