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Pregnant women infected with SARS-CoV-2 during their third trimester are unlikely to pass on the virus or protective antibodies to their newborn, according to two peer-reviewed studies by Harvard Medical School researchers published last month.

The researchers examined two ways in which infected mothers could pass on the virus to their newborns: vertical transmission — the direct passage of the virus from the mother to the newborn immediately before or after birth — and placental infection — or passage of the virus into the placenta.

Although there have been documented occurrences of vertical transmission, the study published in JAMA Network Open found the transmission to be incredibly rare: none of the 64 mothers with Covid-19 enrolled in the study passed the virus onto their newborns.

The team also found that the virus was not detected in the bloodstream of pregnant women infected with SARS-CoV-2, according to the study’s lead author, HMS assistant professor Andrea G. Edlow.

In another study published in Cell, researchers found that unlike influenza and pertussis — two common and highly contagious diseases like Covid-19 — SARS-CoV-2-specific protective antibodies were not transferred from the mother to the newborn.

The researchers were intrigued by the fact that the rate of antibody transfer seemed to be higher for women infected during the second trimester, as opposed to the third.

“We also had a cohort of women that were infected during the second trimester,” said HMS graduate student Caroline G. Atyeo, first author of the Cell paper. “And what we saw is that women that were infected during the second trimester didn’t show this compromised placental antibody transfer.”

Atyeo said she wanted to investigate more deeply what was causing this decrease in antibody transfer; she and her team hypothesized that it was linked to Fc glycosylation, or the attachment of different carbohydrate or sugar groups to antibodies.

Edlow — who was also involved in the Cell study — said that upon becoming infected with the novel coronavirus, the “carbohydrate patterns” on the mothers’ antibodies were altered, decreasing their transfer efficiency.

However, the immune systems of pregnant women were able to compensate for the transfer inefficiency by increasing the total amount of antibodies in their system. The most effective antibodies, which activate “natural killer cells” that destroy the virus, were transferred “preferentially” to the fetus, according to Edlow.

Findings from both studies present questions for future research.

Evan A. Bordt, a postdoctoral fellow at HMS involved in both studies, said it would be important to “keep an eye on” the babies born from mothers infected with Covid-19 during their pregnancy.

According to Edlow, there could be “subtle effects,” such as hyperactivity or learning difficulties, on the neurodevelopment of newborns whose mothers had been infected with Covid-19 in pregnancy, which is true of other viral infections.

Atyeo said these findings may also prove important for SARS-CoV-2 vaccine development and research.

“It might be important for when we’re looking at maternal immunization to look to see if the vaccine actually induces antibodies that can transfer across the placenta efficiently,” she said. “And then I think it’s really important to consider the timing of when these mothers are vaccinated.”

Emory University Gynecology and Obstetrics chair Denise J. Jamieson — who was not involved with either study — said the JAMA Network study “certainly raises concern about whether the vaccine is going to be effective in providing neonatal benefit.”

Both Jamieson and Edlow, however, emphasized that these findings do not mean that pregnant women — who are “more likely to have severe disease, more likely to go to the ICU, and even more likely to be intubated, or to die, than women who are their same age that are not pregnant,” according to Edlow — should not be vaccinated.

“Overall, I’m recommending that pregnant women get vaccinated because of the risks to their health,” Jamieson said. “It would be really nice, and an added benefit, if we could also say that it will help protect your baby after birth, but based on this, we can’t.”

—Staff writer Ariel H. Kim can be reached at ariel.kim@thecrimson.com.

—Staff writer Lauren L. Zhang can be reached at lauren.zhang@thecrimson.com.