COVID-19 vaccination during pregnancy protects babies against Omicron infection

Receiving an mRNA vaccine during pregnancy appears to put newborns at low risk of SARS-CoV-2 infection with the Omicron variant through 6 months of age, according to a study from Singapore.

In a cohort of infants who had at least one parent infected with SARS-CoV-2 within the first 6 months after delivery, the overall estimated vaccine effectiveness was 41.5 percent (95 percent confidence interval [CI], 22.8–55.7) among infants born to mothers who had received their second or third dose during pregnancy. [JAMA Netw Open  2023;6:e2342475]

Vaccine effectiveness was lower if mothers completed their first and second doses only (37.6 percent, 95 percent CI, 17.2–53.1) than if mothers completed their third dose as well (45.5 percent, 95 percent CI, 22.4–61.7). There was no significant difference in vaccine effectiveness by the trimester during which the mothers received a third dose (first: 45.5 percent; second: 43.8 percent; third: 45.0 percent).

However, maternal vaccination conferred no benefit if mothers received their vaccine prior to pregnancy, the investigators pointed out.

Abundance of antibodies

“Infants derive direct protection from maternal vaccination through acquisition of antibodies via the transfer of antibodies across the placenta during pregnancy and consumption of milk via breast feeding. The amount of anti-SARS-CoV-2 antibodies transferred from mother to umbilical cord blood at birth is dependent on the level of antibodies in the mother and this wanes with time,” the investigators explained. [Clin Infect Dis 2023;76:e274-e279; Nat Commun 2022;13:3571]

“High levels of neutralizing antibodies are required for protection against infection by Omicron variants. Hence, mothers who did not receive vaccination during pregnancy are likely to have a lower level of antibodies compared with those who received vaccination during pregnancy,” they continued. [Nat Med 2021;27:1205-1211; Sci Rep 2023;13:17337]

In the present cohort, Omicron XBB variant–specific analysis showed a reduced risk of Omicron XBB infection among infants born to mothers who received their third dose during pregnancy (vaccine effectiveness, 76.7 percent, 95 percent CI, 12.8–93.8).

“Worryingly, we found that receipt of the third dose prior to pregnancy did not overcome this lack of lower risk among infants for SARS-CoV-2 infection (vaccine effectiveness, −0.4 percent, 95 percent CI, −67.5 to 39.8). This requires confirmation from other studies as there was only one SARS-CoV-2 infection among infants whose mother received a third dose prior to pregnancy in the cohort,” the investigators said.

They pointed to the lower level of antibodies as one of the possible reasons for the absence of protection against the Omicron variant infection among infants born to mothers vaccinated before pregnancy seen in the study as well as in an Israeli study. [Nat Med 2023;29:1155-1163]

Need for antenatal vaccination

The study included 7,292 infants, of whom 62.0 percent had mothers who were Chinese, 7.2 percent had mothers who were Indian, 27.5 percent had mothers who were Malay, and 3.2 percent had mothers who were of other ethnicity.

Most infants (97.6 percent) were born to mothers who had been fully vaccinated or boosted as of 14 days prior to delivery, and 17.4 percent contracted COVID-19 during the study period. The crude incidence rate of infection was 174.3 per 100,000 person-days among infants born to mothers who were unvaccinated, 122.2 per 100,000 person-days among infants born to mothers who were vaccinated before pregnancy, and 128.5 per 100,000 person-days among infants born to mothers who were vaccinated during pregnancy.

“Our study, which was designed to reduce bias from differences in SARS-CoV-2 exposure by only including infants of parents who had a confirmed SARS-CoV-2 infection during the first 6 months after delivery, adds to the evidence on the association of maternal vaccination during pregnancy with a lower risk of infants acquiring SARS-CoV-2 infection. In addition, we identified in our Omicron XBB variant–specific analysis the need for a third dose during pregnancy for the lower risk of infection to be maintained,” according to the investigators.

If the present data are confirmed, then “there may be a need for mRNA SARS-CoV-2 vaccination to be recommended for every pregnancy similar to maternal influenza and pertussis vaccination in order to maintain protection in newborns,” they said.