Among 83 babies born to mothers who had antibodies against the coronavirus, 87% tested positive for immunoglobulin G, an antibody created by the immune system to fight off infection.
In most cases, immunoglobulin G develops seven to 10 days after the symptoms of COVID-19 begin and remains in the blood after the infection has resolved, the researchers said.
The presence of the antibody suggests the mothers were infected with COVID-19 in the recent past and developed antibodies that may protect them -- and perhaps their newborns -- from future infection, according to the researchers.
"Our work shows that maternal antibodies to [COVID-19] can efficiently cross the placenta, and therefore demonstrates the potential for such maternally derived antibodies to provide neonatal protection from infection," study co-author Dr. Karen M. Puopolo told UPI.
"To put it simply, when a woman makes antibodies to [the] virus during pregnancy, those antibodies are transferred to her baby," said Puopolo, chief of newborn pediatrics at Pennsylvania Hospital in Philadelphia.
Although the findings suggest that expectant mothers with past COVID-19 infection may be able to pass immunity to their newborns, researchers say it's unclear whether any protection afforded by vaccination against the virus also can be transferred.
"The vaccines currently being given are designed to induce antibodies to the spike protein, so you could infer that vaccine-induced antibody will cross the placenta as well," Puopolo said.
"However, work still needs to be done to determine what levels and types of antibody are needed to protect newborns from infection, and how long those antibodies may last in the newborn circulation," she said.
For this study, researchers at Children's Hospital of Philadelphia and Pennsylvania Hospital tested more than 1,700 pregnant women for COVID-19 antibodies.
Of these women, 83 tested positive for the antibody immunoglobulin G, which indicates past infection, the researchers said.
Once these mothers delivered their babies, the researchers tested blood samples collected from the umbilical cords for COVID-19 antibodies.
Among mothers with antibodies, 72, or 87%, had babies who tested positive for antibody, which means they may have some immunity against the new coronavirus, the data showed.
"We see our studies as part of the first step in understanding how the maternal immune response, whether from infection or immunization, could offer protection to her newborn," Puopolo said.