Although pregnant women’s exclusion from vaccine trials has received considerable attention, lactating women have also been routinely excluded, despite clear clinical and public health reasons for their inclusion. Postpartum vaccination is common, yet data on vaccine safety, antibody transfer through breastmilk, and potential infant immunity remain limited. The theoretical risk from most vaccines is low, but empirical evidence is essential. Lactation alone should not routinely constitute an exclusion criterion for non-live vaccine studies.

Breastfeeding provides critical immunological protection to infants and represents a period of ongoing maternal-infant vulnerability to infectious disease. Ethical inclusion of lactating individuals in vaccine research requires transparent communication about breastmilk antibody transfer, pharmacokinetics, and any theoretical risks, while weighing these against the risks of infant disease exposure without maternal vaccination. 

Vaccine research should actively monitor infant health outcomes when breastfeeding is the primary source of nutrition. Key indicators include gastrointestinal symptoms, infection rates, and any adverse effects observed in the infant. These data are critical for assessing vaccine safety during lactation and informing public health guidance. 

Example: COVID-19 
Early COVID-19 vaccine trials excluded lactating women, leaving an initial evidence gap. Observational studies subsequently demonstrated that mRNA vaccination in lactating women leads to the presence of neutralizing antibodies in breastmilk without harm to the infant. These findings supported real-world recommendations for vaccination during lactation.