Women who receive COVID-19 mRNA vaccines during pregnancy pass high levels of antibodies to their babies, according to a new study published in the American Journal of Obstetrics & Gynecology – Maternal Fetal Medicine.

The effectiveness of mRNA COVID-19 vaccines, the researchers say, lies in their ability to trigger the production of the right antibodies, blood proteins capable of protecting individuals from infection. Whether this protection could be passed from mothers to their newborns had remained a question.

The new study found that 100% of 36 newborns tested at the time of birth had protective antibodies after their mothers received the vaccines produced by either Pfizer-BioNTech or Moderna.

Antibodies can either be produced as part of the natural response to infection or triggered by vaccines. With that in mind, the research team was also able to tell apart antibodies in the neonatal blood in response to natural infection from those made in response to the vaccines. The result becomes relevant, as natural antibody responses against the SARS-COV-2 virus are not sufficiently protective for many people, and with recent data from the Centers for Disease Control and Prevention suggesting that just 23 percent of pregnant women have been vaccinated despite growing evidence of prenatal vaccine safety.

Led by researchers at NYU Grossman School of Medicine, the study authors observed the highest levels of antibodies in cord blood of mothers fully vaccinated in second half of their pregnancy. This insight provides evidence of transferred immunity to neonates, which correlates to protection against infection for infants the first months of life.   

“Studies continue to reinforce the importance of vaccines during pregnancy and their power to protect two lives at once by preventing severe illness in both mothers and babies,” says Ashley S. Roman, MD, director of the Division of Maternal-Fetal Medicine and Silverman Professor of Obstetrics and Gynecology at NYU Langone Health, and one of the study’s principal investigators. “If babies could be born with antibodies, it could protect them in the first several months of their lives, when they are most vulnerable. 

In the current study, though the sample size is small, “it is encouraging that neonatal antibody levels are high if women are vaccinated,” says Jennifer L. Lighter, MD, associate professor in the Department of Pediatrics and hospital epidemiologist at Hassenfeld Children’s Hospital at NYU Langone, and the study’s senior author.   

Existing studies analyze antibodies to the spike protein (anti-S IgG) alone, which may be present after natural infection or vaccination, and do not include antibodies to the nucleocapsid protein (anti-N IgG), which is only present following natural infection. Among the 36 samples collected, all had high levels of anti-S IgG. 31 of those samples were tested for anti-N IgG and were negative.

”High levels of transplacental antibody transfer are not surprising. It is consistent with what we see with other immunizations. Our findings add to a growing list of important reasons why women should be advised to receive the COVID-19 vaccine during pregnancy for the added benefit of their newborn receiving crucial protection,” adds Dr. Lighter. 

Additional research is needed to determine how effective the infant antibodies are, how long protection will last, and if vaccination in the second half of pregnancy may confer higher levels of antibody transfer than vaccination earlier in pregnancy. Future studies should also focus on antibody transmission to newborns in a larger population and durability of antibody detection during infancy.