Long-term Pfizer vaccine-elicited SARS-CoV-2 antibody profile in breast milk

Even as the overall safety of the coronavirus disease 2019 (COVID-19) vaccines in pregnant women appears to be supported by available post-vaccination data, a new study published on the preprint server medRxiv* appears to indicate that the Pfizer vaccine induces antibodies against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These antibodies have been shown to pass into breast milk, thus protecting the baby for at least six months from the date of vaccination.

Study: Quantification and progress over time of specific antibodies against SARS-CoV-2 in breast milk of lactating women vaccinated with BNT162b2 Pfizer-BioNTech COVID-19 vaccine (LacCOVID). Image Credit: evso / Shutterstock.com


The researchers of the current study aimed to understand the antibody titers in the breast milk of women who were breastfeeding at one month from vaccination with the messenger ribonucleic acid (mRNA) Pfizer/BioNTech vaccine against COVID-19. They also wanted to determine whether maternal serum levels of specific antibodies to the virus were correlated with breast milk antibody concentrations.

The authors of the current study also analyzed vaccine efficacy and adverse reactions associated with vaccination in mothers or infants. This is the first long-term study on breast milk antibody titers following the administration of this vaccine.

The current study included 33 mother-infant pairs, at a median age of 38 years for the mothers and 15 months for the infants. None of the study participants tested positive for the infection at the beginning of the study. There were 28 and 32 participants after the first and second dose, respectively, while eight dropped out by six months.

Altogether, there were 149 serum and milk samples each, the first set being taken at a median of two weeks from the first dose, and the second at 14 days from the second dose. The third sample was obtained 28 days from the second dose, and the fourth at 12 weeks, while the last was taken at 24 weeks following the second dose.

Study findings

The investigators found that at two weeks from the second dose, the median anti-spike S1 immunoglobulin G (IgG) antibody levels were highest in both serum and milk, following a parallel course from the earliest time point in their rise and fall, though separated by a large margin.

Adverse events related to vaccination included mostly local injection site pain, which was reported by almost all women, while fever, malaise, and headache were present in about 1% of study participants. Other expected mild systemic symptoms were reported in a small fraction of mothers, with no observable effects in the infants.

Two mothers had breakthrough SARS-CoV-2 infections, one asymptomatic and diagnosed in retrospect, and the other mild. In the first case, IgG-S1 levels were found to be suddenly elevated in both serum and milk compared to the earlier fourth sample, while anti-nucleocapsid antibodies, which are not induced by the vaccine spike antigen, were found in serum but not in milk.


Fears surrounding the potential ability to breastfeed to cause vertical transmission of the virus from infected mothers and, in the case of the vaccine, that it could cause harm to the fetus or baby during pregnancy and breastfeeding, respectively. This led to a paucity of evidence-backed guidelines on breastfeeding practices following vaccination of lactating mothers, or even the advisability of vaccinating this group.

Post-marketing monitoring has been the primary source of data on vaccine safety in this case, as clinical trials of the vaccine did not include women who were pregnant or breastfeeding. This led to the LacCOVID study, which used data from frontline workers who were lactating and had taken the vaccine early on.

The findings from the current study confirm that specific antibody to SARS-CoV-2 pass into the breast milk in vaccinated women, with a close correlation to serum levels, as shown by earlier short-term studies. However, this is the first study to continue for six months.

While antibody levels in serum decline over this period, as expected from currently available data from other research reports, the findings here show that breast milk samples also show declining antibody levels at three and six months, again in parallel with serum levels.

“Therefore, it is reasonable to hypothesize that serum determination of SARS-CoV-2 IgG-S1 could indicate approximately the breastmilk levels of antibodies during the 6 months after vaccination.”

Vaccination using an mRNA vaccine in pregnancy is linked to the transplacental passage of protective antibodies to the virus, thus conferring immunity over the neonatal period. Meanwhile, this study shows that vaccination during the early post-partum period may protect the infant for at least six months.

This conclusion is tempered by the occurrence of two breakthrough infections in this small cohort, both after three months from the date of vaccination, which appears to be a relatively high incidence among healthcare workers. The protective efficacy of these antibodies in the infant thus needs to be analyzed further, as well as the incidence of COVID-19 among lactating women.

The researchers also showed that even at high levels, anti-nucleocapsid IgG fails to appear in breast milk. The study also indicates the potential utility of a booster shot for this group, since the breakthrough infections were, in both cases, linked to a marked rise in antibody titers in serum and breast milk.

Larger samples will be required to validate and extend these findings following the use of mRNA and other vaccines, as well as using different antibody detection techniques. The protective efficacy of these antibodies in breast milk also needs to be assessed.

The current study confirms the safety of the Pfizer vaccine during lactation, as well as the occurrence of specific antibodies in breast milk at six months from vaccination with a late decline. If found to be protective, the vaccination of lactating women may prevent the transmission of the virus among infants, both by group immunity and through breast milk antibodies.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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