Infants were protected against delta and, to some degree, omicron COVID-19 infections during the first 6 months of life when their mothers were immunized with mRNA COVID-19 vaccines, including a second dose during pregnancy, a study shows. “A third vaccine dose bolstered protection against omicron,” the authors write. “Effectiveness for two doses was highest with maternal vaccination in the third trimester, and effectiveness decreased in infants beyond eight weeks of age.”
The test-negative design study was conducted in Ontario and included infants younger than 6 months of age who were born between May 7, 2021, and Mar. 31, 2022. Community and hospital SARS-CoV-2 testing and patterns of delta or omicron infections or hospitalizations among the infants showed the following: “8,809 infants met eligibility criteria, including 99 delta cases (4,365 controls) and 1,501 omicron cases (4,847 controls). Infant vaccine effectiveness from two maternal doses was 95% (95% confidence interval 88% to 98%) against delta infection and 97% (73% to 100%) against infant hospital admission due to delta and 45% (37% to 53%) against omicron infection and 53% (39% to 64%) against hospital admission due to omicron. Vaccine effectiveness for three doses was 73% (61% to 80%) against omicron infection and 80% (64% to 89%) against hospital admission due to omicron. Vaccine effectiveness for two doses against infant omicron infection was highest with the second dose in the third trimester (53% (42% to 62%)) compared with the first (47% (31% to 59%)) or second (37% (24% to 47%)) trimesters. Vaccine effectiveness for two doses against infant omicron infection decreased from 57% (44% to 66%) between birth and eight weeks to 40% (21% to 54%) after 16 weeks of age.”