In this study, 11 people donated breastmilk at various timepoints after having received an mRNA COVID vaccine. The researchers then used two rounds of PCR to allow them to detect extremely small amounts of mRNA. By doing this, they were able to find small amounts (a maximum of 11 parts per trillion - roughly equivalent to a single tear in an Olympic swimming pool) of vaccine mRNA in the whole milk of three of the donors. By spinning the milk very fast to concentrate it, they were able to detect vaccine mRNA in a further two of the eleven milk samples. They were not able to detect any mRNA more than 48 hours post-vaccination.

The findings from this study are in line with those from two published before, which also found that extremely small amounts of vaccine mRNA can be found in the breast milk of some individuals shortly after vaccination. Similar to these two previous studies, the authors conclude that the amount of vaccine mRNA found in milk is sufficiently small that this does not pose a safety concern.