Antibodies against SARS-CoV-2 found in breast milk of women who received Pfizer vaccine

August 20, 2021

2 min read

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Lactating women who received the Pfizer-BioNTech COVID-19 vaccine had specific anti-SARS-CoV-2 antibodies in their breast milk, according to findings published in JAMA Network Open.

Researchers said they found higher quantities of antibodies in breast milk samples that were collected after participants received a second dose of the vaccine.

“It is a small sample size, but the results are consistent,” Vicens Diaz-Brito, MD, PhD, head of the infectious diseases department at Parc Sanitari Sant Joan de Deu at Sant Boi, told Healio Primary Care. “These data suggest that babies breastfed by immunized women could be protected against COVID-19, at least for the duration of the breastfeeding period.”

Diaz-Brito and colleagues conducted a prospective cohort study of 33 lactating women in Spain who received the Pfizer COVID-19 vaccine. All the women included in the study were first-line health care workers, Diaz-Brito said.

The researchers analyzed serum and breast milk samples that were taken from each participant 2 weeks after they received the first dose of the vaccine, 2 weeks after they received the second dose, and again 2 weeks later. None of the participants had a confirmed COVID-19 infection. Their mean age was 37.4 years, and the mean postpartum time was 17.5 months.

The median IgG levels for serum-milk pairs at each time point were 519 (234-937) to 1 (0-2.9) arbitrary units (AU) per mL 2 weeks following the first dose; 18,644 (9,923-29,264) to 78 (33.7-128) AU/mL 2 weeks following the second dose; and 12,478 (6,870-20,801) to 50.4 (24.3-104) AU/mL 4 weeks following the second dose, according to Diaz-Brito and colleagues. No major adverse reactions were observed in mothers or infants, the researchers reported.

“Serum and breast milk vaccine antibodies increased dramatically after the second dose, and remained elevated after 1 month,” Diaz-Brito said.

He noted that there was a “positive correlation between serum and breast milk vaccine antibodies.”

“This fact is important, because we could estimate breast milk antibody values only with a blood determination,” he said. “However, it is necessary to confirm our data in further studies with higher sample sizes.”

Additional studies with different COVID-19 vaccines are underway to investigate levels of immune response and “their passage into breast milk,” Diaz-Brito said.

The researchers also noted that more research is needed to determine whether breast milk antibody levels decrease or plateau after vaccination.

Perspective

Danielle M. Carter, MD

The body of early data demonstrating the passage of antibodies against SARS-CoV-2 through breast milk post-vaccination is very encouraging but not at all surprising. It has long been known that breast milk confers protection against infections, including respiratory tract infections, through secretory IgA antibodies. There is also evidence to suggest that this protection also lasts after cessation of breastfeeding through stimulation of the infant’s immune system. This knowledge is part of the reason other vaccines are recommended during pregnancy and lactation, specifically Tdap and the flu vaccine. More studies will be needed to define the antibody levels that confer immunity specific to COVID-19. However, given these findings and the known safety of mRNA vaccination in pregnancy and lactation, the benefit of vaccination continues to be clear and should be recommended to all lactating individuals.

Danielle M. Carter, MD

Family physician, Ascension St. Vincent’s Riverside

Physician member, American Academy of Family Physicians Board of Directors

Disclosures: Healio Primary Care could not confirm relevant financial disclosures at the time of publication.