A new study by the University of Massachusetts Amherst found that women vaccinated against COVID-19 transfer SARS-CoV-2 antibodies to their breastfed infants, potentially giving their babies passive immunity against the coronavirus.

The research has been published in the Obstetrics and Gynecology Journal. "This research is the first to detect SARS-CoV-2 antibodies in stool samples from infants of vaccinated mothers," said lead author Vignesh Narayanaswamy, a PhD candidate in the breast milk research lab of senior author Kathleen Arcaro, professor of environmental toxicology in the Department of Veterinary and Animal Sciences.

"This is really important because women want to know whether their babies have these antibodies, and our study shows that antibodies are being transferred via breast milk. Providing this compelling evidence is the motivation for women to continue breastfeeding after they receive the vaccine," he added.

Narayanaswamy noted another important takeaway: the antibodies were detected in infants regardless of age - from 1.5 months old to 23 months old. 30 lactating women from across the U.S. - most of the healthcare workers - were enrolled in the study. They received the COVID-19 mRNA vaccine between January and April 2021. The women provided breast milk samples before they were vaccinated, across two to three weeks after their first vaccine dose and across three weeks after the second dose.

They also gave samples of their blood, spotted on cards, 19 days after the first dose and 21 days after the second dose. Infant stool samples were collected 21 days after the mothers' second vaccination. Pre-pandemic samples of breast milk dried blood spots, and infant stools were used as controls for the study.

The samples were tested for a receptor-binding domain (RBD)-specific immunoglobulin (Ig)A and IgG antibodies. In the breast milk samples, anti-RBD IgG antibodies were found to neutralise the protein spike of SARS-CoV-2, as well as four variants. A significant increase in cytokine levels also revealed the immune response in breast milk samples.

Anti-RBD IgG and anti-RBD IgA antibodies were detected in 33 per cent and 30 per cent of infant stool samples, respectively. The levels of antibodies correlated with the vaccine side effects the mother experienced. "Women who did feel sick from the vaccine was associated with greater antibodies in the infant stool," Arcaro said.

"So you might have felt bad, but that was a benefit for your infant," she added. The study, Arcaro said, received no specific funding and was partially driven by the participants themselves, who were familiar with Arcaro's wide-ranging breast milk research, including the New Moms Wellness Study and BRCA gene-mutation research that Narayanaswamy focused on.

While lactating and pregnant women were urged to be vaccinated, no pregnant or breastfeeding women were included in the vaccine trials, Arcaro noted. Findings showed that "even if you had COVID, there is a benefit for women to get the vaccine."

The research team included Arcaro's UMass colleagues Dominique Alfandari, Brian Pentecost and Sallie Schneider; Dr Corina Schoen, assistant professor of obstetrics and gynaecology at UMass Chan Medical School-Baystate; and Ryan Baker, a UMass Amherst undergraduate student.