Three doses of the Moderna mRNA COVID-19 vaccine were more effective against infection with the SARS-CoV-2 Delta variant than against Omicron but were highly protective against hospitalization with either subtype, according to a study yesterday in Nature Medicine.A team led by Kaiser Permanente Southern California researchers conducted a test-negative case-control study among 26,683 COVID-19 cases caused by the Delta or Omicron variants in December 2021.
Of all cases, 16% were Delta, and 84% were Omicron. The incidence of Omicron infections in Southern California increased from 1.2% to 94.1% from Dec 6 to 31.Higher effectiveness against Delta infectionOne dose of vaccine was 56.7% effective (95% 95% confidence interval [CI], 40.7% to 68.4%) against Delta infection and 20.4% effective (95% CI, 9.5% to 30.0%) against Omicron.Vaccine effectiveness (VE) of two doses of the Moderna COVID-19 vaccine against Omicron infection was 44.0% (95% CI, 35.1% to 51.6%) at 14 to 90 days but fell quickly.
Three doses, however, resulted in a VE of 93.7% (95% CI, 92.2% to 94.9%) against Delta at 14 to 60 days and 86.0% (95% CI, 69.7% to 73.4%) after 60 days.Against Omicron infection, VE after three doses was 71.6% (95% CI, 69.7% to 73.4%) at 14 to 60 days and 47.4% (95% CI, 40.5% to 53.5%) after 60 days.Three doses were 29.4% effective (95% CI, 0.3% to 50.0%) against Omicron infection among 30 people with compromised immune systems, compared with 75.5% in their non-immunocompromised counterparts.
An analysis that excluded people with impaired immune systems showed a three-dose VE against Omicron infection of 51.2% (95% CI, 44.2% to 57.3%) after 60 days.The full three-dose vaccine regimen was 70.9% effective (95% CI, 68.9% to 72.9%) against