Women who receive COVID-19 mRNA vaccines produced by Pfizer-BioNTech or Moderna while in their third trimester of pregnancy generate a strong immune response and pass protective antibodies through umbilical cord blood to their babies, according to a study conducted by Weill Cornell Medicine and NewYork-Presbyterian researchers, published April 28 in Obstetrics & Gynecology.

Researchers studied 122 women who received the two-dose Pfizer or Moderna mRNA vaccine during pregnancy and analyzed the antibody response mounted upon vaccination. They also assessed the presence of antibodies in the cord blood of babies born to these women at the time of birth. The research demonstrated that 99% of newborns had protective antibodies after their mothers received both vaccine doses, and 44% of babies had antibodies after one dose. 

“Receiving two doses prior to birth will increase the likelihood that antibodies are transferred to the baby,” said lead study author Dr. Malavika Prabhu, associate director of labor and delivery at Weill Cornell Medicine and at NewYork-Presbyterian Alexandra Cohen Hospital for Women and Newborns. “This finding is encouraging for pregnant women who want to protect both themselves and their newborn against COVID-19.”

The researchers also found that vaccination led to antibody production in pregnant women as early as five days after the first dose, while the transfer of antibodies through the placenta to the baby occurred as early as 16 days after the first dose. The researchers also observed that the higher the maternal antibody levels, the higher the newborn antibody levels, and that the chances of having detectable antibodies in the mother and in their neonates increase with time elapsed since vaccination.

“The study suggests that women who are pregnant shouldn’t delay getting both vaccine doses if they have access to vaccination,” said senior study author Dr. Yawei Jenny Yang, assistant professor of pathology and laboratory medicine at Weill Cornell Medicine and an assistant attending pathologist at NewYork-Presbyterian/Weill Cornell Medical Center. “Ensuring an adequate amount of time between vaccination and birth ensures that the pregnant woman has enough time to mount an antibody response as well as enough time to pass antibodies on through the cord blood.”

“It is encouraging to see the transplacental transfer of antibodies against COVID-19 to babies. However, we do not know what level of antibody is protective against COVID-19, and how long those protective antibodies will last, for either the mother or the baby,” Prabhu said. “For now, the safest way to protect newborns from COVID-19 is continued hand hygiene, social distancing and mask usage.”

The researchers are conducting further studies to understand the variability in antibody transfer to the baby in the womb and why this transfer does not always occur. They also want to determine the best timing for vaccination, whether this is in the first, second or third trimester and how breastfeeding impacts antibody levels in babies. They are currently conducting a longer-term follow-up study, so they can understand the protective nature of antibodies from the vaccine on the mother and the baby.

“Pregnant women were excluded from initial clinical trials for COVID-19 vaccines, so many women had to make decisions for themselves and their baby without data to turn to,” said Yang, who is also assistant director of the Central Laboratory at Weill Cornell Medicine and NewYork-Presbyterian/Weill Cornell Medical Center. “We hope this new research gives these women important data to guide them as well as greater peace of mind in their decisions.”

Heather Lindsay is a freelance writer for Weill Cornell Medicine.