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Sources: American Society for Biochemistry and Molecular Biology; American Society for Biochemistry and Molecular Biology
The main components of breast milk include the sugar lactose, lipids or fats, proteins and carbohydrates such as human mil koligosaccharides, or HMOs. Breastfeeding is often recommended for mothers as these components largely support a growing babies’ nutrition and provide antibodies against viral and bacterial germs. For example, HMOs are thought to play an important role in the developing gut microbiome.
The Academy of Breastfeeding Medicine does not recommend cessation of breastfeeding for individuals who are vaccinated against COVID-19. Individuals who are lactating should discuss the risks and benefits of vaccination with their health care provider, within the context of their risk of contracting COVID-19 and of developing severe disease. Health care providers should use shared decision making in discussing the benefits of the vaccine for preventing COVID-19 and its complications, the risks to mother and child of cessation of breastfeeding, and the biological plausibility of vaccine risks and benefits to the breastfed child.
The vaccine is made of lipid nanoparticles that contain mRNA for the SARS-CoV-2 spike protein; the mRNA sequence only encodes this protein. These particles are injected into muscle, where the nanoparticles are taken up by muscle cells. These muscle cells then transcribe the mRNA to produce spike protein. The spike protein made by the cell stimulates an immune response, protecting the individual from COVID-19 illness.
During lactation, it is unlikely that the vaccine lipid would enter the blood stream and reach breast tissue. If it does, it is even less likely that either the intact nanoparticle or mRNA transfer into milk. In the unlikely event that mRNA is present in milk, it would be expected to be digested by the child and would be unlikely to have any biological effects.