Babies Less Likely to Catch COVID When Born to Vaccinated Moms

Infants born to mothers who were vaccinated against COVID-19 during pregnancy had a lower risk of testing positive for SARS-CoV-2, even when the Omicron variant was dominant, Norwegian researchers found.

During the Omicron period, infants whose mothers received a second or third dose of COVID vaccine during pregnancy had a 33% lower risk of a positive test at age 4 months than infants born to unvaccinated mothers (HR 0.67, 95% CI 0.57-0.79), reported Ellen Øen Carlsen, MD, of the Norwegian Institute of Public Health in Oslo, and colleagues.

The differences between infants with vaccinated versus unvaccinated mothers were even more dramatic during the Delta period, where the risk of a positive test was 71% lower (HR 0.29, 95% CI 0.19-0.46), the authors wrote in JAMA Internal Medicine.

Recent studies showed that vaccination during pregnancy helped protect infants from serious illness and risk of hospitalization from COVID, but did not focus on protection from infection.

Carlsen and co-authors examined data from Norwegian birth registries from September 2021 to February 2022, and used the Norwegian Immunization Register to derive data on COVID vaccines. Women who received a second or third dose of mRNA vaccine, such as Pfizer or Moderna, after 83 gestational days and up to 14 days prior to delivery were considered vaccinated, the team noted.

Overall, 21,643 newborns were included, and 45% of those were born to women who were vaccinated during the last two trimesters of pregnancy. Unvaccinated mothers tended to be younger, with more children and a lower education level, the researchers said.

There were 906 infants who tested positive for COVID via a PCR test within the first 4 months of life, for an incidence rate of 5.8 per 10,000 follow-up days. During the Delta period (defined as before Jan. 1, 2022), incidence rates were 1.2 and 3.0 per 10,000 for infants born to vaccinated and unvaccinated mothers, respectively. Incidence rates for the Omicron period were 7.0 and 10.9 per 10,000 follow-up days. Sensitivity analyses focusing mainly on the Omicron period produced similar results.

There were also 2,839 infants born to mothers who only received one dose of mRNA vaccine during pregnancy. Of those, 36 had a positive COVID test, including fewer than five infants during the Delta period. During the Omicron period, adjusted HR for a positive test among these infants versus those born to unvaccinated mothers was 0.72 (95% CI 0.50-1.03).

Interestingly, the authors noted that among 824 infants whose mothers had a third dose of COVID vaccine during pregnancy, none had a positive test during Delta, and risk of a positive test during Omicron was lower for those with a third dose (aHR 0.22, 95% CI 0.12-0.43) versus those with two doses (aHR 0.70, 95% CI 0.59-0.83).

Limitations to the data include lack of specific information about the variant that each infant tested positive for, and that vaccines during the last 14 days of pregnancy or postpartum were not included. The authors also acknowledged the potential for residual confounders.

"The findings of this study provide early evidence to suggest that infants benefit from passive protection from SARS-CoV-2 infection following maternal COVID-19 vaccination during pregnancy," the researchers concluded.

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