This research letter shares findings from a study of 11 lactating individuals vaccinated with either Moderna (mRNA-1273) or Pfizer-BioNTech (BNT162b2) mRNA vaccines. Trace amounts of the vaccines were detected in seven breast-milk samples from five participants within 45 hours of vaccination. These findings demonstrate that vaccine mRNA can reach mammary cells and be packaged into extracellular vesicles (EVs), potentially allowing transport to other cells. Supporting evidence from animal studies also shows that in rats, up to three days after intramuscular injection, low levels of vaccine mRNA were found in the heart, lung, testis, and brain, indicating systemic tissue distribution. The authors speculate that lipid nanoparticles may also reach mammary glands via blood or lymphatic routes before being secreted in milk through EVs.
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