Evidence Builds That Pregnant Women Pass Covid Antibodies to Newborns

One of the many big questions scientists are trying to untangle is whether people who get Covid-19 during pregnancy will pass on some natural immunity to their newborns.

Recent studies have hinted that they might. And new findings, published Friday in the journal JAMA Pediatrics, provide another piece of the puzzle, offering more evidence that Covid-19 antibodies can cross the placenta.

“What we have found is fairly consistent with what we have learned from studies of other viruses,” said Scott E. Hensley, an associate professor of microbiology at the Perelman School of Medicine at the University of Pennsylvania and one of the senior authors of the study.

Additionally, he added, the study suggests that women are not only transferring antibodies to their fetuses, but also transferring more antibodies to their babies if they are infected earlier in their pregnancies. This might have implications for when women should be vaccinated against Covid-19, Dr. Hensley said, adding that vaccinating women earlier in pregnancy might offer more protective benefits, “but studies actually analyzing vaccination among pregnant women need to be completed.”

In the study, researchers from Pennsylvania tested more than 1,500 women who gave birth at Pennsylvania Hospital in Philadelphia between April and August of last year. Of those, 83 women were found to have Covid-19 antibodies — and after they gave birth, 72 of those babies tested positive for Covid-19 antibodies via their cord blood, regardless of whether their mothers had symptoms.

According to Dr. Karen Puopolo, an associate professor of pediatrics at the University of Pennsylvania and one of the senior authors of the study, about half of those babies had antibody levels that were as high or higher than those found in their mother’s blood, and in about a quarter of the cases, the antibody levels in the cord blood was 1.5 to 2 times higher than the mother’s concentrations.

“That’s fairly efficient,” Dr. Puopolo said.

The researchers also observed that the longer the time period between the start of a pregnant woman’s Covid-19 infection and her delivery, the more antibodies were transferred, a finding that has been noted elsewhere.

The antibodies that crossed the placenta were immunoglobulin G, or IgG, antibodies, the type that are made days after getting infected and are thought to offer long-term protection against the coronavirus.

None of the babies in this study were found to have immunoglobulin M, or IgM, antibodies, which are typically only detected soon after an infection, suggesting that the babies hadn’t been infected with the coronavirus.

Experts don’t yet know if the amount of antibodies that passed on to the babies were enough to prevent newborns from getting Covid-19. And because only a few of the babies in the study were born prematurely, the researchers can’t say whether babies who are born early might miss out on those protective antibodies. The study authors also noted that because their results were from just one facility, the findings would need to be further replicated.

The placenta is a complex organ, and one that has been understudied, said Dr. Denise Jamieson, an obstetrician at Emory University in Atlanta and a member of the Covid expert group at the American College of Obstetricians and Gynecologists, who was not involved with the study.

And more research is needed to better understand whether vaccine-generated antibodies behave comparably to antibodies from Covid-19 infection, said Dr. Andrea G. Edlow, an assistant professor of obstetrics, gynecology and reproductive biology at Harvard Medical School.

In a study published in the journal Cell in December, for instance, Dr. Edlow and her colleagues found that Covid-19 antibodies from a natural infection might cross the placenta less efficiently than the antibodies produced after vaccination for flu and whooping cough (pertussis).

“What we really want to know is, do antibodies from the vaccine efficiently cross the placenta and protect the baby, the way we know happens in influenza and pertussis,” Dr. Jamieson said.

Experts do not know whether the Covid vaccine will work in this way, in part because pregnant women were excluded from the initial clinical trials.

“It’s plausible that the Covid vaccine will offer protection to both pregnant mothers and their infants,” said Dr. Mark Turrentine, a member of the Covid expert group at A.C.O.G. “To me,” he added, “this study highlights that inclusion of pregnant women in clinical trials such as the Covid-19 vaccine is essential, particularly when the benefit of vaccination is greater than the potential risk of a life-threatening disease.”