You’ve probably seen those awesome photos of two containers of expressed breast milk side-by-side. Usually, the one expressed while baby is healthy is lighter in color, nearly white, and the other, expressed when baby has a cold or other infection, is much more yellow. The difference in color comes from antibodies, molecules that your immune system makes in response to invaders like bacteria and viruses. Those antibodies can attach to bacteria and viruses, which serves as a signal that alerts the cells of your immune system to destroy the invaders.
In a study published in 2020 in Frontiers in Pediatrics, researchers showed that multiple types of antibodies are present in breast milk and that they are high in the early days, when your body is making colostrum, and persist during extended breastfeeding.  Over the past year and a half or so with the rise of the COVID-19 pandemic, researchers have been particularly interested in looking into the presence of COVID-19 antibodies in breast milk, after both infection and vaccination. In this blog post, we’ll discuss the evidence for protection from breast milk against COVID-19.
In April, Sivan Haia Perl, a pulmonary physician at Shamir Medical Center in Israel, and colleagues published a study in the Journal of the American Medical Association describing the identification of antibodies against the virus that causes COVID-19, SARS-CoV-2, in healthcare workers who were breastfeeding a child.  They determined that more than 80 percent of the 504 samples of breast milk that they tested contained antibodies for SARS-CoV-2 four weeks after the initial vaccination and one week after the second dose of the Pfizer/BioNTech COVID-19 vaccine.
A group of researchers at the University of Florida led by neonatologist Vivan Valcarce Luaces tested breast milk from healthcare workers who received either the Pfizer/BioNTech or Moderna COVID-19 vaccine and reported the results in a study published in August in the journal Breastfeeding Medicine. They found COVID-19 antibodies in the breast milk samples from all 21 participants. Importantly, there did not appear to be a difference in the Pfizer/BioNTech and Moderna COVID-19 vaccines in terms of antibody content of breastmilk, meaning both vaccines are likely good options for breastfeeding people. 
COVID-19 antibodies also make it into breast milk after SARS-CoV-2 infections, even if they are asymptomatic. Rebecca Powell, an infectious disease researcher at the Icahn School of Medicine at Mount Sinai in New York, and colleagues showed in a paper published in October of 2020 in the journal iScience that breast milk from all 15 people who had confirmed COVID-19 or experienced COVID-19 symptoms and then recovered had SARS-CoV-2 antibodies. 
And another study, published in mBio this February, showed that the majority of breast milk samples obtained from COVID-19 patients was able to neutralize the SARS-CoV-2 virus when mixed together in a dish, suggesting that human milk contains functional antibodies against the virus. That study, which was led by Ryan Pace of the University of Idaho, also found no evidence of SARS-CoV-2 transmission via breast milk, which is good news for anyone who wants to keep breastfeeding, but might have been exposed. 
In addition to the studies mentioned above, other work has confirmed that human milk is likely to contain SARS-CoV-2 antibodies both after vaccination and after infection with the virus. What researchers know about studying antibodies to other germs that are found in breast milk is that they are likely still effective after passing through baby’s digestive tract, and thus could help protect breastfeeding babies and toddlers from severe COVID-19 illness. If you’re pregnant or breastfeeding and still on the fence about getting vaccinated, check out any of the helpful posts here, particularly this one about why vaccination is better than infection.
- Czosnykowska-Łukacka et al., “Changes in Human Milk Immunoglobulin Profile During Prolonged Lactation,” Frontiers in Pediatrics, 2020.
- H. Perl et al., “SARS-CoV-2–Specific Antibodies in Breast Milk After COVID-19 Vaccination of Breastfeeding Women,” JAMA, 2021.
- V. Valcarce et al., “Detection of SARS-CoV-2-Specific IgA in the Human Milk of COVID-19 Vaccinated Lactating Health Care Workers,” Breastfeeding Medicine, 2021.
- Fox et al., “Robust and Specific Secretory IgA Against SARS-CoV-2 Detected in Human Milk,”
- M. Pace et al., “Characterization of SARS-CoV-2 RNA, Antibodies, and Neutralizing Capacity in Milk Produced by Women with COVID-19,” mBio, 2021.