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Maternal exposures during lactation influence the composition of a woman’s breast milk. Out of concern that the mRNA in mRNA-based COVID-19 vaccines could pass into breast milk and alter infant immunological development, many lactating women have declined vaccination against COVID-19 or have stopped breastfeeding prior to vaccination. Findings from a new study indicate that mRNA from an mRNA-based vaccine does not pass into breast milk.

mRNA-based vaccines contain the genetic instructions for synthesis of a single viral protein that, when injected into the body, stimulates the immune system to make antibodies against a specific target. The mRNA is housed in lipid nanoparticles to protect it, and because mRNA can’t enter a cell’s nucleus, concerns about its safety are low. Robust evidence indicates that the mRNA-based vaccines against COVID-19 are highly effective, and a small, prospective cohort study found that maternal vaccination may provide protection against COVID-19 in breastfed infants.

The current study involved seven lactating women (average age, 37 years) who were scheduled to receive either the Pfizer BioNTech or Moderna mRNA-based vaccines against COVID-19. The women provided breast milk samples prior to vaccination and between four and 48 hours afterward. The samples were chilled or frozen until they were analyzed for mRNA content via real-time quantitative polymerase chain reaction assay.

Analysis of the breast milk samples revealed that none of the samples contained detectable levels of vaccine mRNA. The authors of the study posited that if small quantities of mRNA were present in the breast milk (below the detection limits of their assay), they would likely be broken down in the infant gut. Although this was a very small study, these findings suggest that mRNA-based vaccines against COVID-19 are safe for lactating women and their infants, but further study in larger groups is warranted.

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