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Study shows breastfeeding mothers can pass C19 vaccine mRNA to babies

  • A study has shown that breastfeeding mothers can pass trace amounts of Covid 19 vaccine mRNA to babies through breast milk.

The study was published on September 26 by international peer reviewed Jama Pediatrics, the oldest continuously published paediatric journal in the United States.

It involved eleven breastfeeding women ranging in age from 22 to 38.

Six women had received the Pfizer C19 mRNA product and five received Moderna.

Samples of expressed breast milk were collected before vaccination and for five days post vaccination and the study involved a total of 131 milk samples collected from one hour to five days after vaccine administration.

The results found trace amounts of the Pfizer and Moderna mRNA products in seven samples from five different women up to 45 hours after vaccination. The study found no Covid 19 mRNA detected in samples beyond 48 hours of vaccination.

The mRNA was found to be present in the whole expressed breast milk sample, in the fat, in the cells and in extracellular vesicles (EVs)  – which are small nanoparticles with lipid membranes naturally released by cells.

The study is the first demonstration of the distribution of the C19 vaccine mRNA to mammary cells with potential to travel elsewhere in the body, the study states.

“These data demonstrate for the first time to our knowledge the biodistribution of COVID-19 vaccine mRNA to mammary cells and the potential ability of tissue EVs to package the vaccine mRNA that can be transported to distant cells. Little has been reported on lipid nanoparticle biodistribution and localization in human tissues after COVID-19 mRNA vaccination.”

“In rats, up to 3 days following intramuscular administration, low vaccine mRNA levels were detected in the heart, lung, testis, and brain tissues, indicating tissue biodistribution,” the discussion report states.

The study notes that its limitations include the relatively small sample size and the study did not test the possible cumulative vaccine mRNA exposure after frequent breastfeeding in infants.

The authors concluded that they believe it is safe to breastfeed after maternal C19 vaccination but urged caution in breastfeeding children under six months in the first 48 hours after maternal vaccination ‘until more studies are conducted.’

The study further noted that ‘potential interference’ of COVID-19 vaccine mRNA with multiple routine other vaccines given to infants in their first six months of life needs to be considered.

“It is critical that lactating individuals be included in future vaccination trials to better evaluate the effect of mRNA vaccines on lactation outcomes,” the report states.

Current HSE advice states that both pregnant women and breastfeeding women should ‘get vaccinated to protect yourself from serious illness with COVID-19.’


In specific HSE advice to breastfeeding mothers, the HSE advice states:

“You can continue to breastfeed safely after being vaccinated.”

“COVID-19 vaccines do not affect breastfed babies. There is no known reason to avoid breastfeeding if you are vaccinated.”


Media officers at the Health Products Regulatory Authority (HPRA), Department of Health and the Royal College of Physicians Ireland (RCPI) had not responded to a query regarding acknowledgement of the study or plans to update public health advice at the time of publication.

The HSE said that “the paper which was only recently published, is being reviewed”.

“The HSE only uses vaccines when they meet the required standards of safety and effectiveness and after the European Medicines Agency  (EMA) has licensed them.”

“The Health Products Regulatory Authority (HPRA) and the European Medicines Agency (EMA) monitor COVID-19 vaccines for safety and effectiveness.”

Covid 19 mRNA products are not currently available to children under five in Ireland.


This story was updated on 28.09.2022 at 18.26 to include a further response from the HSE. 

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