For a study, researchers sought to assess immune responses to coronavirus disease 2019 (COVID-19) mRNA-based vaccinations found in breast milk, as well as immune response transmission to nursing babies. The cohort research included 30 lactating mothers who received mRNA-based COVID-19 vaccinations from January to April 2021. Women gave serial milk samples, including milk produced prior to immunization, 2–3 weeks after the first dose, and 3 weeks after the second dose. Women gave their blood, which was observed on cards (dried blood spots), 19 days after the first and 21 days after the second doses. Stool samples were taken from breastfed babies 21 days following their mothers’ second immunization. As controls, pre-pandemic samples of milk, dried blood spots, and newborn feces were employed. The enzyme-linked immunosorbent assay was used to look for the receptor-binding domain (RBD)–specific immunoglobulin (Ig)A and IgG in milk, dried blood spots, and newborn feces. Neutralizing antibodies against the spike and 4 variations of concern were found in milk samples: D614G, Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1). In milk samples, the levels of 10 cytokines were evaluated.

The spike and 4 variations of concern were neutralized by milk from COVID-19-immunized women, which were predominantly driven by anti-RBD IgG. The immunological response in milk includes a considerable increase in interferon-. The immunological response to maternal immunization was mirrored in breastfed infants, with anti-RBD IgG and anti-RBD IgA found in 33% and 30% of newborn stool samples, respectively. Anti-RBD antibody levels in newborn feces were associated with maternal vaccination adverse effects. In pre-pandemic milk and newborn stool samples, median antibody levels against RBD were lower than the positive cutoffs.

Most women’s breast milk contains humoral and cellular immune responses to mRNA-based COVID-19 immunization. Milk anti-RBD antibodies are capable of neutralizing the SARS-CoV-2 spike and variants of concern. Breastfed newborns get anti-RBD antibodies, which have the ability to offer passive protection against SARS-CoV-2.