Pregnant women with COVID-19 are at an increased risk for severe illness and death compared to people with COVID-19 who are not pregnant, and they experience preterm birth and pregnancy loss more frequently than do expecting moms who don’t catch the virus. In spite of these risks, there is no clear guidance available yet for vaccinating pregnant women against COVID-19. But there is now some evidence that immunization could protect their newborns. For the first time, doctors report that SARS-CoV-2 antibodies from a vaccinated mother can cross the placenta, pointing to a likely benefit for her fetus.

“It validates what we’ve always suspected . . . that mothers can be vaccinated during pregnancy and can provide some of that level of immunity to their unborn child,” says Chad Rudnick, a pediatrician at Florida Atlantic University and one of the authors of the study, which...

His case report documents the experience of a front-line healthcare worker who opted to receive her first dose of the Moderna mRNA COVID-19 vaccine while 36 weeks pregnant. The mother was considered not to have had COVID-19 previously because she had no history of exposure or symptoms, although she was not tested for SARS-CoV-2 antibodies prior to vaccination. After the birth of her baby at 39 weeks, Rudnick detected IgG antibodies against SARS-CoV-2 in the cord blood, confirming the transfer of maternal antibodies to the fetus via the placenta.

Rudnick says that he and coauthor Paul Gilbert, a pediatrician at Florida Atlantic, had not planned to do a study on COVID-19 vaccination in a pregnant woman, but when the opportunity presented itself, they recognized it as a chance to get a first look at how vaccine immunity might be passed along to a fetus.

See “Breastmilk Harbors Antibodies to SARS-CoV-2

Vaccines for influenza and tetanus, diphtheria, and pertussis (TDaP) are commonly used in pregnancy because of their protective effects for newborns, who have few of their own immune defenses. Rudnick says he hopes the same will be true of COVID-19 vaccines, “because we do know that little babies can get hospitalized from COVID. They can sometimes have severe illness.” 

The study did not quantify the levels of antibodies in the maternal blood, so there’s no indication of the efficiency of antibody transfer, and the researchers can’t say whether the antibodies in the infant were protective. Also, because it was a case study of only one subject, the paper was not able to suggest the ideal timing of maternal vaccination.

COVID-19 vaccination for pregnant women

A recent publication in JAMA Pediatrics showing that SARS-CoV-2 antibodies can similarly cross the placenta during natural infection offers some clues about the optimal timing window for vaccination. Karen Puopolo, a neonatologist at the Children’s Hospital of Philadelphia, and colleagues found SARS-CoV-2 IgG antibodies in the placentas from 72 out of 83 pregnant women who had previously had the virus. The concentration of antibodies in the cord blood correlated with maternal antibody concentration and the time between infection and delivery. Specifically, if the mom’s antibodies levels were high, so were the levels in the cord blood, and the longer before delivery that she had the infection, the greater the abundance of antibodies. 

“That kind of makes sense for a new disease,” says Puopolo. “It takes some time for your body to make an antibody response, and then it takes some time for that antibody to be transferred across the placenta.” Her team found that 17 days before birth was the minimum time for maternal SARS-CoV-2 antibodies to show up in the cord blood after infection. That same window may apply to vaccination as well, she says.    

See “Human Fetuses Can Contract SARS-CoV-2, but It’s Rare

These results highlight an ongoing debate about the inclusion of pregnant women in clinical studies, including trials of COVID-19 vaccines. According to Puopolo, the reluctance to experiment on pregnant women is grounded in good intentions, “but it can be taken too far, and when it’s taken so far that pregnant women and their newborns may not be able to benefit from research that could be helpful to them, then we’ve gone too far.” 

Geeta Swamy, who studies maternal immunization at Duke University, agrees that pregnant women should have been included in clinical trials for COVID-19 vaccines. “The problem now is that we’re going to be faced with the potential difficulties in enrolling women in studies when they could be getting the vaccine already.” That’s because pregnant women can receive the vaccines that have emergency use authorization if they fall into the eligible groups, while in a placebo-controlled study, participants typically have a 50 percent chance of receiving the placebo.

Swamy is preparing to begin an observational study of COVID-19 vaccination in pregnant women with the Centers for Disease Control and Prevention (CDC). She is a member of the US Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee, although she does not participate in any COVID-related decisions due to her involvement as a coinvestigator for Pfizer and BioNTech’s COVID-19 vaccine trial, which did not accept pregnant women, at the Duke clinical trial site.

Currently, the most authoritative guidance for COVID-19 vaccinations for pregnant women comes from the American College of Obstetricians and Gynecologists, which advises that the vaccine should not be withheld due to pregnancy, and that women should make their own decision after consulting with their physician. While awaiting more data about the risks and benefits of the vaccine during pregnancy, these early studies suggest that a vaccination could provide newborns with some level of protection against infection.

P. Gilbert, C. Rudnick, “Newborn antibodies to SARS-CoV-2 detected in cord blood after maternal vaccination,” medRxiv, doi:10.1101/2021.02.03.21250579, 2021.

Correction (February 18): The article previously stated that the mother who received the vaccine had tested negative for antibodies before her immunization, but she was not tested. We have clarified that, based on her lack of exposure and symptoms, the doctors considered her to not have had COVID-19. The Scientist regrets the error.

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