SAN FRANCISCO (CBS SF) — Hoping to ease fears among nursing mothers, UCSF researchers released a study Friday that found no evidence of COVID-19 vaccine appearing in breast milk after a mother receives a COVID-19 vaccination.

Dr. Stephanie L. Gaw, one of the study’s authors, said the research showed there was no threat to newborns if their mothers are vaccinated.

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“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,” Dr. Gaw said in a news release.

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.

The study analyzed the Pfizer and Moderna vaccines, both of which contain mRNA.

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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.

“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.

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The study authors noted that it was limited by the small sample size and said that further clinical data from larger populations were needed.