COVID-19 Vaccination During Pregnancy Found to Protect Infants Against Infection, Hospital Admission


The efficacy of 2 doses against Omicron infection was highest when the second vaccine dose was given in the third trimester of pregnancy compared with the first or second trimesters.

New research has found that 2 doses of mRNA COVID-19 vaccine during pregnancy are highly effective against infection with the Delta variant and moderately effective against the Omicron variant. Vaccination during pregnancy is also linked to a lower risk of hospital admission in infants under 6 months of age.

According to the study findings, protection against Omicron infection was greatest when a mother received a second vaccine dose in the later stages of pregnancy and was also greatest for infants in their first 8 weeks of life. Receiving a third (booster) dose during pregnancy also strengthened protection against the Omicron variant.

COVID-19 vaccines are not yet approved for infants under 6 months of age, but a growing body of evidence suggests that vaccination during pregnancy may reduce the risk of infection and hospital admission in infants. This occurs when antibodies are transferred across the placenta and through breast milk, a process known as passive immunity.

To examine this further, investigators aimed to estimate the efficacy of maternal mRNA COVID-19 vaccination during pregnancy against the Delta and Omicron variants and hospital admission in infants. The study involved infants younger than 6 months of age who were born in Ontario, Canada, between May 7, 2021, and March 31, 2022, and who received a polymerase chain reaction (PCR) test for COVID-19 infection between May 7, 2021, and September 5, 2022.

These data were then linked to data on the mothers’ mRNA COVID-19 vaccination status during pregnancy, including whether they were unvaccinated or received 1, 2, or 3 doses. The data were also linked with recorded Delta- and Omicron-related hospital admissions in infants.

Multiple potentially influential factors were taken into account, including the mother’s age at delivery, number of previous pregnancies, pre-pregnancy conditions, and the sex of the infant. In total, 8809 infants were included in the main analysis, with 99 positive Delta COVID-19 cases and 1501 positive Omicron cases.

The results show that 2 vaccine doses during pregnancy were 95% effective against Delta infection in infants and 97% effective against infant hospital admission due to the Delta variant. The effectiveness of 2 doses against the Omicron variant (45%) and admission to hospital (53% was moderate, but improved to 73% and 80%, respectively, with a third dose.

The efficacy of 2 doses against Omicron infection was highest when the second dose was given in the third trimester of pregnancy (53%) compared with the first (47%) or second (37%) trimesters. The efficacy of 2 doses against Omicron infection waned over time, from 57% between birth and 8 weeks of age to 40% after 16 weeks of age.

In addition to the 8809 infants in the main analysis, 421 infants were born to mothers who received only their first vaccine dose during pregnancy, which offered less protection against infection in infants compared with 2 or 3 doses. Specifically, 1 dose offered 81% protection against the Delta variant and 30% against Omicron.

Importantly, the study was observational, so it cannot establish cause and the researchers could not rule out the possibility that unmeasured factors, such as breastfeeding, could have differed between infants of vaccinated and unvaccinated mothers. However, they used detailed information on hospital deliveries, mother’s vaccination status, and PCR test results, and findings were similar after further analysis, suggesting that they are likely to be robust.

The authors also pointed out that 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 overall results concur with previous studies from 4 different countries and are therefore reassuring, the authors wrote in a linked editorial. However, in the rapidly changing landscape of COVID-19 treatment and prevention, even solid conclusions cannot provide definitive answers to many practical questions. Therefore, the authors said more studies are needed to better inform vaccination recommendations.


Covid vaccination in pregnancy protects infants against infection and hospital admission. News release. EurekAlert; February 8, 2023. Accessed February 24, 2023.

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