The study, which analyzed the breast milk of seven women after they received the mRNA vaccines and found no trace of the vaccine, offers the first direct data of vaccine safety during breastfeeding and could allay concerns among those who have declined vaccination or discontinued breastfeeding due to concern that vaccination might alter human milk. The paper appears in JAMA Pediatrics.

Research has demonstrated that vaccines with mRNA inhibit transmission of the virus that causes COVID-19. The study analyzed the Pfizer and Moderna vaccines, both of which contain mRNA.

The World Health Organization recommends that breastfeeding people be vaccinated, and the Academy of Breastfeeding Medicine has said there is little risk of vaccine nanoparticles or mRNA entering breast tissue or being transferred to milk, which theoretically could affect infant immunity.

“The results strengthen current recommendations that the mRNA vaccines are safe in lactation, and that lactating individuals who receive the COVID vaccine should not stop breastfeeding,” said corresponding author Stephanie L. Gaw, MD, PhD, assistant professor of Maternal-Fetal Medicine at UCSF.

“We didn’t detect the vaccine associated mRNA in any of the milk samples tested,” said lead author Yarden Golan, PhD, a postdoctoral fellow at UCSF. “These findings provide an experimental evidence regarding the safety of the use of mRNA-based vaccines during lactation.”

The study was conducted from December 2020 to February 2021. The mothers’ mean age was 37.8 years and their children ranged in age from one month to three years. Milk samples were collected prior to vaccination and at various times up to 48 hours after vaccination.

Researchers found that none of the samples showed detectable levels of vaccine mRNA in any component of the milk.

The authors noted that the study was limited by the small sample size and said that further clinical data from larger populations were needed to better estimate the effect of the vaccines on lactation outcomes.

Co-authors are Mary Prahl, MD; Arianna Cassidy, MD; Christine Y. Lin, BA; Nadav Ahituv, PhD; and Valerie J. Flaherman, MD, MPH, all of UCSF.

The study was supported by the Marino Family Foundation; the National Institutes of Health (grant numbers K23AI127886 and K08AI141728); the Weizmann Institute of Science-National Postdoctoral Award Program for Advancing Women in Science; the International Society for Research in Human Milk and Lactation Trainee Bridge Fund; and the Human Frontier Science Program. Disclosures can be found in the paper.