Pregnant women who receive a coronavirus vaccine not only acquire protective antibodies against the virus for themselves but also may pass along immunity to their babies, emerging research shows.

Several preliminary studies suggest that women who received an mRNA vaccine such as Moderna or Pfizer during pregnancy had covid-19 antibodies in their umbilical cord blood. One of the studies also detected antibodies in their breastmilk, indicating that at least some immunity could be transferred to babies both before and after birth.

Brenna Hughes, vice chair for obstetrics and quality at Duke University, said several recent preprints, which are papers that have not yet been peer-reviewed, are “the first to show what we had hoped would be true, which is that these vaccines could be potentially protective through antibodies passed on to the fetus.”

“So worries about possible risk and harm may be proven quite the opposite. In fact, it may be proven that the vaccines actually provide protection to the developing fetus,” said Hughes, who is also co-chair of the American College of Obstetricians and Gynecologists’ covid-19 task force. She was not involved in the studies.

Researchers have already seen that pregnant women who recover from covid-19, the disease caused by the coronavirus, can pass along their natural immunity to their babies. But the observation that vaccine-induced antibodies may reach a fetus through cord blood and a newborn through breastmilk, is a new discovery that may have broader implications in the fight against the virus.

One preprint, which was published this month, studied more than 130 vaccinated women — 84 of whom were pregnant and 31 lactating. It found that these women had similar immune responses to the vaccine as nonpregnant women, suggesting that the vaccine would be just as effective for them.

It also showed that in 10 women who delivered their babies during the study, all had detectable antibodies in their cord blood, and nearly all of the lactating women had antibodies in their breastmilk. One woman who had received only one dose of a two-dose vaccine when she gave birth, also had antibodies in the cord, but it was a lower amount than in those who were fully vaccinated.

Andrea Edlow, who co-authored the study, said one interesting aspect was that the researchers were able to compare antibody responses between the Moderna and Pfizer vaccines. They found that the women who had received the Moderna vaccine had higher levels of one type of antibody, called IgA. This mucosal antibody is typically found in breastmilk and protects breastfed babies against diseases, such as covid-19, that attack mucosal areas, particularly the respiratory tract.

“The significance of generating a more robust IgA response is not totally clear,” said Edlow, assistant professor of obstetrics and gynecology at Harvard Medical School and maternal-fetal medicine attending physician at Massachusetts General Hospital.

But, she added, “if moms are getting a better IgA boost from the Moderna vaccine, could that somehow translate into improved protection of their baby through breastmilk? This is a question that needs to be examined.”

Edlow said the researchers will soon study the effects of the Johnson & Johnson vaccine.

Another preprint, out of Israel, discovered similar results in 20 women who were vaccinated during pregnancy.

And a case study published Monday in the journal BMC Pediatrics found that a pregnant woman who had received one dose of a two-dose mRNA vaccine also had detectable antibodies in her umbilical cord blood upon giving birth. The authors of that study, clinical pediatricians Paul Gilbert and Chad Rudnick, said that if babies could be born with antibodies, it could protect them in the first several months of their lives, when they are most vulnerable.

Experts say these early studies are an important start in measuring the possible transfer of immunity. The next step is to determine how effective the infant antibodies are, and how long the protection will last.

“Overall, I think the message is good news,” said Denise Jamieson, professor and chair of the department of gynecology and obstetrics at Emory University and a member of the American College of Obstetricians and Gynecologists’ covid-19 task force.

Jamieson, who was not involved in the studies, said it will be interesting to see whether coronavirus vaccines given to pregnant women could protect newborns in the same way that influenza and DTaP vaccinations do. But “it’s also important to remember that the main reason why we’re so focused on getting pregnant women vaccinated for covid is because we know the mothers are at increased risk of severe disease,” she said.

Which, in turn, puts babies an at increased risk, Jamieson added.

The Centers for Disease Control and Prevention lists pregnancy as a risk factor for severe illness and complications from covid-19, including preterm birth.

Edlow said such studies are critical because pregnant women and children are among some of the most vulnerable groups for respiratory illnesses such as covid-19. And yet, they are some of the least studied and the last to be eligible for novel therapies and vaccines, she said.

“So any studies that focus on these populations, especially during the pandemic, are going to be important,” she said.

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