Women who are vaccinated during pregnancy may not only protect themselves from COVID-19 but also their babies, according to new research published Tuesday by the Centers for Disease Control and Prevention.

The study found that infants under 6 months old were 61% less likely to be hospitalized with COVID-19 if their mothers had received a Pfizer or Moderna vaccination against the disease during pregnancy, and concluded that vaccination during pregnancy “might help prevent COVID-19 hospitalization among infants.”

“The data CDC is publishing today provides real world evidence that getting a COVID-19 vaccine during pregnancy might protect infants,” said Dr. Dana Meaney-Delman, chief of the CDC’s infant outcomes monitoring research and prevention branch.

The authors noted that there is evidence from other vaccine-preventable diseases that maternal immunization can provide protection to infants through what they call “passive transplacental antibody transfer.”

But despite recent studies suggesting this occurs with COVID-19 vaccination during pregnancy, there has been no epidemiologic evidence until now.

The study involved 20 pediatric hospitals in 17 states that assessed effectiveness of pregnant womens’ vaccinationsin preventing COVID-19 hospitalization in infants from July 1, 2021 to January 17, 2022. The pregnant women had completed a two-dose primary vaccination series. Among 379 hospitalized infants under 6 months old studied, 176 had COVID-19 and 203 did not.

The study had a number of limitations. Lead author Dr. Manish Patel noted that it is “an epidemiologic study” and not a randomized clinical trial, “which allows strongest evidence.”

The study also did not look at any benefits to the infants of mothers who were vaccinated before becoming pregnant, or who had and recovered from COVID-19 either before or during pregnancy, which also could result in antibodies passed on to the baby.

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The study involved only mothers who received the Pfizer and Moderna vaccines using the new mRNA technology, not those who had the Johnson & Johnson shot, and did not have enough evidence on the effect of booster doses or breastfeeding.

The study also did not determine how the infants who had COVID-19 became infected, or account for possible differences in behavior of vaccinated and unvaccinated mothers.