Many studies involving hundreds of women and their infants have been reported in the literature. No evidence suggests that 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.[1-4] Neutralizing capacity may increase even while antibody levels drop.[5] Nursing mothers experience minimal disruption of breastfeeding after vaccination, which may be related to having systemic symptoms.[6] A few mothers have reported to blue or blue-green discoloration of their milk.[7-11] 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.[12-22]
Only a small percentage of milk samples from women who received an mRNA vaccine contain trace amounts of mRNA. mRNA has not been detected in the serum of any breastfed infants.[23-26] mRNA has an estimated serum half-life of 8 to 10 hours and is not detected in milk beyond 48 hours.[23,24,26,27] Moreover, the mRNA in milk is inactive in producing an immune response.[27] 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.[28] 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 vaccine had higher milk antibody levels than those who had either only an infection or two doses of vaccine during pregnancy.[29,30] Pregnancy may increase the anti-spike IgA and IgM antibodies in milk in a second pregnancy after prior vaccination of COVID-19 infection.[31] 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.[32] Milk antibody levels persist for at least 6 to 8 months after vaccination. There appear to be no major differences in antibody response from the Pfizer-BioNTech and Moderna vaccines, although some studies found a better IgA response to the Moderna vaccine than the Pfizer-BioNTech vaccine.[33-35] Vaccine-induced clones are unique to individuals.[36] 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.[33,37-40] One study found the weakest breastmilk antibody response to the CanSino vaccine, compared to the Janssen and Pfizer-BioNTech vaccine.[40]
A booster of the Pfizer-BioNTech or Moderna vaccine markedly increases IgG milk titers, including following an initial vector vaccine, but IgA titers are affected variably.[41-46] A vaccination interval of 3 to 6 weeks appears to generate a better milk antibody response than at 6 to 16 week intervals between vaccinations.[35] Women who had both a SARS-CoV-2 infection and vaccination with an adenovirus vector vaccine (Sputnik V [Gamaleya Institute] or ChAdOx1-S [Astra-Zeneca]) had higher IgA and IgG levels than women who received only a vector vaccine. Women vaccinated with BIBP-CorV (Sinopharm) had similar IgA levels in milk as with the vector vaccine, but lower milk IgG levels.[47]
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.[48,49] 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.[50]
Most lactating persons who received an mRNA vaccine booster reported no adverse effects on lactation or other obstetric concerns.[51] In an on-line survey of persons who received a COVID-19 vaccine (56% Pfizer-BioNTech, 15% AstraZeneca, 9% Moderna), 52 were nursing women with an autoimmune disease. Side effects they experienced were no more frequent than those of healthy control women.[52] A review of adverse reaction reported to the World Health Organization found an increased risk of mastitis in nursing mothers after unspecified (probably various types) COVID-19 vaccines.[53] Some women have reported a small increase in mean menstrual cycle length for cycles in which participants received the first dose (0.5 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.[54] Another on-line survey of comprising 184 women in Columbia, some reported changes in frequency, regularity duration and volume of menses.[55] Women using a menstrual cycle tracking app reported a less than one day adjusted increase in the length of their first and second vaccine cycles. The change in menstrual cycle length was temporary and there was no change in menses length. The type of vaccine used did not affect the outcome;[56] Reports submitted to The Netherlands Pharmacovigilance Center found 41.4 reports of menstrual abnormalities per 1000 women who received a COVID-19 vaccine. Amenorrhea or oligomenorrhoea and heavy menstrual bleeding collectively accounted for about half of all abnormalities reported.[57]