Vaccines against COVID-19 were not detected in human milk, according to a small study, indicating vaccine safety for pregnant and lactating women and providing early evidence that the shots are not transferred to the infant.

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Researchers from the University of California-San Francisco (UCSF) analysed the breast milk of seven women after they received the mRNA vaccines -- Pfizer and Moderna -- and found no trace of the vaccines that are known to inhibit transmission of SARS-CoV2, a virus that causes COVID-19.

The study, detailed in 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 World Health Organisation 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 Stephanie L. Gaw, Assistant Professor of Maternal-Foetal Medicine at UCSF.

"We didn't detect the vaccine associated with mRNA in any of the milk samples tested," said lead author Yarden Golan, a postdoctoral fellow at UCSF. "These findings provide 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 ranging 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. However, 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.