Vaccinated Mothers Pass COVID-19 Antibodies to Breastfeeding Infants

COVID-19 vaccination is approved for children 5 years and older, but younger children and babies are also susceptible to infection. A recent study, published in the Obstetrics & Gynecology journal, found that breastfeeding babies can receive COVID-19 antibodies from their vaccinated mothers, giving the babies passive immunity against the virus.

The cohort study included 30 lactating women who were fully vaccinated with 2 doses of an mRNA COVID-19 vaccine between January-April 2021. Women were enrolled from across the continental US, and a minimum of 10 or more women received each vaccine (Pfizer-BioNTech or Moderna).

The participants completed questionnaires assessing their demographics, lactation status, general health, previous COVID-19 infection history, type of vaccine received, timing of inoculation, and any maternal or infant adverse events.

The women provided serial milk samples before vaccination, 2-3 weeks after their first dose, and 3 weeks after their second. Investigators collected dried blood spots from the mothers 19 days after the first vaccine and 21 days after the second. Infant stool samples were collected 21 days after the mothers received their second vaccine dose.

As a control, the investigators analyzed samples of milk, blood, and infant stool collected pre-COVID-19 pandemic. All samples were tested via enzyme-linked immunosorbent assay for receptor-binding domain (RBD) immunoglobulin (Ig)A and IgG. Additionally, milk samples were tested for neutralizing antibodies against the spike protein of 4 COVID-19 variants of concern: D614G, Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1).

The results showed that milk from the vaccinated women neutralized the spike and 4 variants of concern, primarily due to the presence of anti-RBD IgG. The milk also had a significant elevation of interferon-γ.

Immune response to maternal vaccination was confirmed by the infant stool samples. Anti-RBD IgG was detected in 33% of samples, and anti-RBD IgA were detected in 30%. Notably, the levels of anti-RBD antibodies in infant stool correlated with their mothers’ side effects.

Kathleen Arcaro, senior author and a professor of environmental toxicology in the Department of Veterinary and Animal Sciences at the University of Massachusetts Amherst, said, “Women who did feel sick from the vaccine was associated with greater antibodies in the infant stool. So you might have felt badly, but that was a benefit for your infant.”

The investigators concluded that humoral and cellular immune responses to inoculation with an mRNA COVID-19 vaccine were present in the breast milk of most women (87%). Arcaro commented, “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 motivation for women to continue breastfeeding after they receive the vaccine.”

Given that vaccination is strongly recommended for pregnant women, the passing of COVID-19 antibodies from mother to baby is a positive indication of the potential for COVID-19 immunity.