Babies’ Immune Response After Maternal Infection With SARS-CoV-2

New research reveals that women who test positive for COVID-19 at the time they give birth pass along antibodies in breast milk, protecting their newborns from infection

When women test positive for SARS-CoV-2 shortly before giving birth, what happens to their babies? A new study out of Rome suggests that breastfeeding, along with specific pregnancy-related biological processes, offers a measure of protection to babies and prevents them from becoming infected with the virus.

A team of scientists led by Rita Carsetti, MD, a physician at Bambino Gesu Children’s Hospital in Rome, recruited 28 women who tested positive for COVID-19 and then gave birth at a clinic in Rome between November 2020 and May 2021. The researchers took blood samples from both mothers and infants 48 hours after birth and then again 2 months after birth to test for antibodies to the virus. They collected samples of breast milk within 3 days of delivery from some of the participants, and again at 2 months postpartum if the mother was still lactating. They also took saliva samples from the newborns 48 hours after birth and again at 2 months old.

At the study’s end, 21 women and 22 babies (including a set of twins) were still enrolled. The scientists found that while some of the mothers had begun building IgG, IgA, and IgM antibodies to COVID-19 in their blood at the time of birth, only 1 baby had blood antibodies passed on by the mother. At 2 months after birth, nearly all the mothers had seen their antibody levels rise significantly, and 3 babies had detectable IgG, IgA, and IgM antibodies.

While other research demonstrates that pregnant women with SARS-CoV-2 infection can and do transmit antibodies to their fetuses through the placenta, in this case babies’ antibody levels weren’t significant due to the timing of their mothers’ infections. Carsetti explained that it takes at least 2 weeks for antibody production to even begin. Because the women were infected with the virus so close to the births of their babies, “[t]hey didn’t have enough time to make [antibodies] and give them to the baby through the placenta,” she said. “That does not happen in a day.” Had the women been infected with or vaccinated against the virus 2 or 3 months before delivery, she added, they would have had time to produce antibodies that could pass through the placenta.

But while serum antibodies were not detected in most babies, probably due to the timing of their mothers’ COVID-19 infections, IgA antibodies were detected in all breast milk samples and were higher 48 hours after birth than 2 months later. Not only were antibodies found in breast milk, they also were found in the babies’ saliva. As Carsetti explained it, the antibodies in breast milk stimulate a mucosal immune response in infants, allowing them to generate their own antibodies. “It’s a genius idea of evolution,” she said. A control group of babies who were not breastfed showed no COVID-19 antibodies in their saliva.

It’s been known for decades that breastfeeding allows mothers to bestow on their babies immunity to various types of infections, and SARS-CoV-2 appears to be no exception. Carsetti suggested that clinicians advocate breastfeeding should a mother find herself testing positive for Covid-19 at the time of birth, as this will prevent the baby from getting infected. Ideally, she said, all pregnant women will be vaccinated in their third trimester to allow ample time for antibodies to develop and travel through the placenta to the fetus.

In general, babies are not at high risk of COVID-19. Breastfeeding is not the only factor that protects babies, Carsetti noted--but as with many things related to COVID-19, there are unknowns. “We don’t know how long this antibody production in the children lasts,” Carsetti said, adding that most maternal antibodies to other infections that are passed to the baby wane after about 3 months. To gather more information, the research team is continuing to follow the babies in the study, who are now about a year old.

The study has a few limitations. The sample size was small and drew from a single healthcare facility. Six mothers who had positive test results had no symptoms, making it difficult to determine exactly how long they had been infected. Finally, the researchers did not have a control group throughout the study because they didn’t want to subject uninfected mothers and babies to invasive sample collection procedures.