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.