Researchers have found no evidence for the presence of Pfizer or Moderna COVID-19 vaccines in human breast milk in a small study, suggesting that the mRNA preventives are safe during lactation. The study, published in the journal JAMA Pediatrics, 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 researchers at University of California, San Francisco (USCF) in the US analysed the breast milk of seven women after they received the Pfizer and Moderna mRNA vaccines, and found no trace of the preventives. Previous research has demonstrated that vaccines with mRNA inhibit transmission of the virus that causes COVID-19.
The World Health Organization (WHO) recommends that breastfeeding people be vaccinated, the researchers noted. According to the Academy of Breastfeeding Medicine, there is little risk of vaccine nanoparticles or mRNA entering breast tissue or being transferred to milk, which theoretically could affect infant immunity, they added.
“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 study corresponding author Stephanie L Gaw, an assistant professor at UCSF. “We didn’t detect the vaccine associated mRNA in any of the milk samples tested," said study lead author Yarden Golan, a postdoctoral fellow at UCSF.
The study, conducted from December 2020 to February 2021, provides an experimental evidence regarding the safety of the use of mRNA-based vaccines during lactation. The mothers’ mean age in the study 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 study was limited by the small sample size, the researchers noted, adding that further clinical data from larger populations were needed to better estimate the effect of the vaccines on lactation outcomes.