Pregnant women can reduce their infant's risk of COVID-related hospitalization by receiving at least two doses of an mRNA vaccine, which are highly effective against maternal Delta variant infection and moderately effective against maternal and infant Omicron infection, suggests a study published yesterday in BMJ. A third (booster) dose offered added protection against Omicron.
The University of Toronto–led study used a test-negative design to estimate rates of maternal COVID-19 infection with the Delta or Omicron strains, infant Omicron infection, and infant Delta and Omicron hospitalization among vaccinated and unvaccinated pregnant women tested for infection in Ontario.
Included infants were younger than 6 months from May 7, 2021, to Mar 31, 2022, and were tested for COVID-19 from May 7, 2021, and Sep 5, 2022. Of the 8,809 eligible infants, 99 were infected with Delta, and 1,501 had Omicron infections, with respective uninfected control groups of 4,365 and 4,847.
Highest effectiveness in third trimester
Among the 99 Delta-infected infants, 29% were hospitalized, as were 22% of 1,501 Omicron-infected infants. Estimated two-dose COVID-19 vaccine effectiveness (VE) was 95% against maternal Delta infection and 97% against infant Delta hospitalization. VE was 45% against maternal Omicron infection and 53% against infant Omicron hospitalization.
Three vaccine doses were 73% effective against maternal Omicron infection and 80% effective against infant Omicron hospitalization. Two-dose VE against infant Omicron infection waned from 57% from birth to 8 weeks to 40% after 16 weeks.
Two-dose VE against infant Omicron infection was higher when the second dose was given in the third trimester of pregnancy (53%) than in the first (47%) or second (37%) trimesters. This finding, the authors said, means that the benefits of vaccination need to be weighed against the risks of delayed vaccination to the woman and fetus.
Two-dose VE against infant Omicron infection waned from 57% from birth to 8 weeks to 40% after 16 weeks.
In a subanalysis of 421 infants born to mothers who received only one vaccine dose during pregnancy and no doses before conception or between birth and 14 days before their infant was tested for COVID-19, 75% of vaccinations were received in the third trimester. VE of one dose was 81% against Delta and 30% against Omicron.
Results confirm value of maternal vaccination
"mRNA vaccines are highly effective at preventing severe infection in pregnant women, [who] have an elevated risk of covid-19 complications compared with their non-pregnant counterparts," the researchers wrote. "Moreover, epidemiological studies suggest that adverse pregnancy outcomes, including spontaneous abortion, stillbirth, and preterm birth, after a COVID-19 vaccination during pregnancy are similar to or lower than background rates in the general population."
In a related commentary, Dana Danino, MD, of Ben-Gurion University, and Ilan Youngster, MD, of Tel Aviv University, said that the placental transfer of SARS-CoV-2 antibodies to the fetus is critical to infant protection against COVID-19.
They said that although the study reinforces the value of maternal COVID-19 vaccination, "more studies are needed to better inform vaccination recommendations in an evolving landscape of new SARS-CoV-2 strains and novel vaccines."
