Many studies involving hundreds of women and their infants have been reported in the literature. No evidence suggests that women receiving a vaccine against SARS-CoV-2 is harmful to either the nursing mother or the breastfed infant. Antibodies and T-cells that neutralize the SARS-CoV-2 virus appear in the milk after maternal vaccination.[1-3] Nursing mothers experienced minimal disruption of breastfeeding after vaccination although a few reported to blue or blue-green discoloration of their milk.[4-8] A small percentage of breastfed infants may experience sleepiness, increased fussiness, fever, rash or self-limiting diarrhea, but no serious adverse effects have been reported. Numerous professional organizations and governmental health authorities recommend that COVID-19 vaccines be offered to those who are breastfeeding because the potential benefits of maternal vaccination during lactation outweigh any theoretical risks.[9-16]
Several vaccines for COVID-19, have been developed. Vaccines available in the US (by Pfizer-BioNTech [Comirnaty] and Moderna) are messenger RNA (mRNA) vaccines. Another mRNA vaccine is available in Europe (CureVac). Other vaccines (by Janssen-Johnson & Johnson, Astra-Zeneca, Sputnik-V, and CanSino) are made using human and primate adenovirus vectors. A third type of vaccine available outside of the US is an inactivated whole-virus SARS-CoV-2 vaccine (by Bharat Biotech, Sinopharm and Sinovac). The Novavax vaccine contains a synthetic version of the S protein plus an adjuvant. No non-live vaccine has previously been reported to cause infant adverse effects via breastfeeding.
Only a small percentage of milk samples from women who received an mRNA vaccine contained trace amounts of mRNA. Thirty-six of 40 milk samples in one study and 5 of 309 milk samples in another had detectable mRNA levels; the highest concentration found was 2 mcg/L in one study and the median concentration was 70 ng/L in another; mRNA has not been detected in the serum of any breastfed infants.[18-20] mRNA has an estimated serum half-life of 8 to 10 hours.[18,19] The tiny amount of polyethylene glycol-2000 in Pfizer-BioNTech vaccine is not found in breastmilk or absorbed orally, so breastmilk PEG exposure from maternal immunization is not a concern. Neither of the mRNA vaccines available in the US contains a preservative or adjuvant.
Mothers who receive an mRNA vaccine have marked increases in milk antibodies that are similar to or higher than after a COVID-19 infection. Mothers who had a COVID-19 infection during pregnancy and received a single dose of the Pfizer-BioNTech vaccine postpartum had higher milk antibody levels than those who had only an infection or two doses of vaccine during pregnancy in one small study. Milk IgA antibodies develop within 1 to 2 weeks after the first dose, with a loss in activity of 25 to 30% against the Alpha, Beta and Delta variants relative to the original strain. Milk IgG antibody levels are slower to develop after the first dose of an mRNA vaccine in lactating women, but increase after the second dose and persist in milk longer than IgA. In one study, mothers who had lactated for 24 months or longer had more than double the concentration of anti-viral IgG in their milk than mothers who had breastfed for less than 24 months. There appear to be no major differences in antibody response from the Pfizer-BioNTech and Moderna vaccines, although one study found a better IgA response to the Moderna vaccine than the Pfizer-BioNTech vaccine. Milk antibody response against SARS CoV-2 following the adenovirus vector and inactivated vaccines appear to be considerably weaker than and delayed compared to the mRNA vaccines.[25-29] One study found the weakest breastmilk antibody response to the CanSino vaccine, compared to the Janssen and Pfizer-BioNTech vaccine. Some infants have anti-SARS-CoV-2 IgG in their saliva and stool samples after breastfeeding, and although some gastric and intestinal digestion occurs, titers appear sufficient to neutralize SARS-CoV-2.[30,31] Saliva antibodies potentially protect breastfed infants from infection by coating respiratory surfaces. No increase in serum anti-SARS-CoV-2 antibodies are found in infant serum after maternal vaccination unless mothers were vaccinated during pregnancy. The IgG in milk may offer protection to infants against coronaviruses that cause the common cold. A booster of the Pfizer-BioNTech vaccine markedly increases IgG milk titers, but IgA titers are affected variably.[33,34]
In one on-line survey, some women reported a small increase in mean menstrual cycle length for cycles in which participants received the first dose (0.50 days) and cycles in which participants received the second dose (0.39 days) of mRNA vaccines compared with pre-vaccination cycles. Cycles in which the single dose of Johnson & Johnson was administered were, on average, 1.26 days longer than pre-vaccination cycles. Another on-line survey of comprising 184 women in Columbia some reported changes in frequency, regularity duration and volume of menses. Most lactating persons who received an mRNA vaccine booster reported no adverse effects on lactation or other obstetric concerns.