TORONTO -- New research has found that pregnant mothers transfer fewer antibodies to their infants through the placenta when they have contracted COVID-19, compared to the level of immunity they transfer to infants when infected with similar viruses.

Researchers with the Massachusetts General Hospital (MGH) looked at the immune responses of 22 mothers who tested positive for SARS-CoV-2 -- the virus that causes COVID-19 -- in the third trimester of their pregnancy, as well as 34 mothers enrolled at the same time who were of similar ages, but who had tested negative for SARS-CoV-2.

They found that while mothers with influenza and whooping cough (pertussis) “efficiently” transferred antibodies that protected against these viruses to their infants, it was a different story when it came to COVID-19.

“In contrast, we observed significantly decreased transfer of SARS-CoV-2-specific [antibodies] across multiple SARS-CoV-2 specificities compared to [influenza],” the research, published this month in the journal Cell, stated.

In earlier research, the team had found that it was extremely rare for pregnant people who had tested positive for COVID-19 to transfer the virus itself to their infant, but this new research suggests that newborns born after their mother contacted COVID-19 will not already have a significant number of antibodies that protect against the virus.

Why was this? Researchers believe it has something to do with a process called glycosylation, in which a carbohydrate attaches to proteins or other organic molecules, changing the structure.

“The carbohydrate attachments on SARS-CoV-2-specific antibodies in maternal blood were different than those seen on influenza- and pertussis-specific antibodies,” a press release explained. “This carbohydrate pattern may cause the COVID-specific antibodies to be ‘stuck’ in the maternal circulation, rather than transferred across the placenta via placental antibody receptors.”

Some antibodies were still able to be transferred to infants because of the high level of antibodies within the mother’s body, or if there were more receptors that attracted the SARS-CoV-2 antibodies. The antibodies that did make it through tended to be quite strong, according to the researchers.

Due to the relatively low number of antibodies that are transferred, researchers believe newborns could be particularly at risk of severe cases of COVID-19, since they lack the level of maternally-transferred immunity they usually receive for other viruses.

Those who are pregnant are often among the last to receive vaccines, simply because of heightened safety procedures around them. Researchers believe these findings have implications for how vaccines given to those who are pregnant should be structured.

Co-Senior author Andrea Edlow, a maternal-fetal specialist with MGH and an assistant professor at Harvard Medical School, suggested in the release that the optimal vaccine to aid pregnant people and their newborns would be a vaccine that can boost the levels of specific COVID-19 antibodies, which are structured in a way that allows them to transfer successfully through the placenta.

"We are beginning to define the rules of placental antibody transfer of SARS-CoV-2 for the very first time -- catalyzing our ability to rationally design vaccines to protect pregnant women and their newborns,” added co-senior author Galit Alter, of MIT and Harvard.

The researchers acknowledged that the study is limited by its small number of participants, and added that further research is needed.