COVID-19 Vaccine Study

Breastfeeding mothers who have been inoculated against COVID-19 needn’t worry about any potential adverse effects of the coronavirus vaccine on their breastmilk, a recent study involving University of Idaho researchers concludes.

The findings are significant given that many new mothers have avoided getting vaccinated, worried their milk composition could change or that components of the vaccine might enter their milk and somehow harm their babies.

Results of the study, published in the December 2024 issue of The Journal of Nutrition, are based on laboratory testing of human milk samples from women who received the Pfizer, Moderna or Johnson & Johnson COVID-19 vaccines and women who were infected with the coronavirus.

The researchers found no evidence of any vaccine components entering human milk. They confirmed the Pfizer and Moderna vaccines caused no significant changes in milk composition, with the Johnson & Johnson vaccine causing very subtle, short-lived changes. Furthermore, milk composition of women who contracted the coronavirus but not vaccinated was altered far more.

U of I researchers involved in the project include Shelley McGuire, a professor specializing in maternal and infant nutrition and director of the Margaret Ritchie School of Family and Consumer Sciences; Mark McGuire, a university distinguished professor in the Department of Animal, Veterinary and Food Sciences (AVFS); and Janet Williams, a senior research scientist in AVFS.

“The bottom line is we found absolutely no vaccine components in the milk,” Shelley McGuire said. “We found some effects of the Johnson & Johnson vaccine on milk composition, however there were more substantial changes when women actually got COVID-19.”

The senior author of the paper, Rebecca Powell, an assistant professor at the Icahn School of Medicine at Mount Sinai, New York, supplied milk samples from vaccinated women who hadn’t contracted COVID-19. U of I provided milk samples from unvaccinated women who had contracted COVID-19.

Researchers from the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, which had federal COVID-19 research dollars available to fund the study and analyzed the samples, were also involved in the project. The researchers submitted the initial project proposal to PNNL in December of 2020, about the time that the vaccines were being made widely available.

The three U of I researchers were previously involved in separate research that allayed concerns that the coronavirus could be transmitted to newborns through breastmilk. In fact, they found the breastmilk of women who have contracted the coronavirus is an important source of COVID-19 antibodies.

“We went from one global panic in breastfeeding women to a new global panic in breastfeeding women,” Shelley McGuire said. “If women are worried about their milk composition changing, they should try to avoid getting COVID-19 rather than avoiding the vaccine.”

In October 2022, Shelley McGuire became the first faculty member inducted into the National Academy of Medicine while employed at an Idaho institution. She is also director of a National Institutes of Health Center of Biomedical Research Excellence (COBRE) focused on furthering studies on nutrition and women’s health.

Published in Catching Up with CALS