COVID-19 Booster Boosts Virus-Neutralizing Antibodies in Breast Milk, Small Study Finds

Between December 2020 and May 2022, researchers at the University of Florida collected milk, blood and stool samples from breastfeeding mothers and their babies, at enrollment and at seven follow-up sessions. The question behind the work was narrow: when a mother gets a COVID-19 booster, do the antibodies reach her infant through breast milk, and can they actually neutralize the virus?

The study, published in Frontiers, began with 39 breastfeeding mothers and 25 infants. Incomplete data left 34 mothers and 24 infants in the final analysis. Fourteen of the mothers received an mRNA booster during the study and served as the cases. The vaccinated mothers who did not get a booster were the comparison group. Participants also filled out questionnaires on demographics, family and medical history, and vaccine side effects. Most of the mothers were non-Hispanic White women in their mid-30s, and the infants averaged about 10 months old.

The backdrop is a gap in protection. COVID-19 has killed almost 7 million people since late 2019 and left hundreds of millions debilitated. Vaccines blunted much of that harm, but infants are too young for conventional vaccination and their immune systems are still developing. Maternal vaccination during breastfeeding is already recommended by the US Centers for Disease Control and Prevention against whooping cough, RSV and influenza, and more recently against COVID-19, though the evidence for the last was limited to nonexistent.

The same research group had earlier detected SARS-CoV-2 IgG and IgA antibodies in human milk and infant fecal matter. Halasa and colleagues then confirmed in 2022 that maternal vaccination benefited infants. Later work showed the protection fades, with antibody concentrations falling substantially about six months after vaccination. What remained unknown was whether boosters, which sustain adult immunity, do the same for antibodies passed through milk.

The measurements suggest they do. Boosting substantially raised IgG levels in milk, plasma and infant stool. IgA stayed above pre-vaccination levels but rose far less. The authors wrote that over the 12 months the IgA response was less dynamic, and that the booster mainly drove IgG, a shift toward stronger IgG-mediated protection. IgG concentrations in milk and plasma were correlated, so one could stand in for the other when a sample is unavailable. IgG levels also declined progressively with time after the booster.

The team then tested whether the antibodies work. They used VSV-gfp-SARS-CoV-2-S-gp, a pseudovirus carrying the SARS-CoV-2 spike protein, against baby hamster kidney cells engineered to express the human ACE2 receptor. They measured cell survival with an MTT assay and confirmed it with modified plaque reduction assays. Neutralization improved by an estimated 60% for milk-derived IgG and 90% for plasma-derived IgG after boosting. When IgG was removed from milk samples, their neutralizing power dropped significantly, pointing to IgG as the active component. The authors wrote that cells treated with boosted plasma or milk stayed alive and metabolically active during infection.

The authors conclude that boosters can more than offset the time-dependent decline in IgG, as seen in plasma, milk and infant stool. They also say future studies are needed to explain exactly how IgG in the gut lumen protects infants. The caveats are plain: the sample is small, only 14 mothers were boosted, and the neutralization tests were done in the lab, not in babies. Whether the antibodies prevent infant illness remains for later work to show.