COVID-19 vaccines are not approved for infants, but breast milk may be an alternative way to receive severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies.

Now, a new medRxiv* preprint study finds mothers vaccinated either one of the messenger RNA (mRNA) vaccines transferred neutralizing antibodies and cytokines, including interferon-gamma (IFNγ), to a breastfeeding child. Additionally, the humoral response in breastmilk was effective in neutralizing four variants of concern.

The study’s results suggest SARS-CoV-2 antibodies through breastmilk may help infants develop passive immunity against the virus.

How they did it

The researchers measured immunoglobulin A (IgA) and immunoglobulin G (IgG) neutralizing antibody responses in breastmilk before and after vaccination. They also assessed any changes in cytokine levels in breastmilk.

Stool samples of infants were collected to measure IgA and IgG antibody levels.

The study enrolled 30 women who were lactating and who either received an mRNA-based vaccine or were scheduled to get it (20 received Pfizer-BioNTech, 10 received Moderna). About 27 women self-identified as White, 1 as Black, and 2 as Asian. The study population ranged from 26 to 46 years, and infants ranged from 7 days to 21.7 months.

Of the 30 women, 3 had recovered from a previous SARS-CoV-2 infection.

Participants froze breastmilk samples at several time points after vaccination and donated them to UMass Amherst for analysis. Breast milk samples two days before their first dose were also collected.

Blood samples on sample cards were collected 19 days after the first dose and 21 days after the second dose.

Participants also donated their infants’ stool samples 21 days after they received their second vaccine dose.

To study changes in breast milk after vaccination, the researchers measured antibody levels from donated breast milk and blood samples from a different study before the pandemic. Additionally, the researchers collected stool samples from 6 breastfed children during August 2019.

Side effects from vaccination reported in both mother and child

Vaccine side effects were observed after the first and second vaccine doses, but most side effects occurred after the second.

Women frequently reported vaccine-related fevers (27%), aches (27%), tiredness (37%), or headaches (30%) after the second dose. However, about 73% of mothers reported that their child had no vaccine-related side effects. If there were, it was either fever and/or a runny nose.

IgA and IgG antibody levels found in breastmilk and serum

Two weeks after the second dose, 26/30 breast milk samples showed detectable levels of IgG antibodies specific for the SARS-CoV-2 receptor binding domain.

About 3 of 20 women showed a two- to four-fold increase in IgG antibody levels in their breastmilk after their first Pfizer-BioNTech dose. Conversely, women administered Moderna did not show signs of antibodies until after the second vaccine dose.

About 14 women had their breastmilk tested positive for IgA antibodies.

Irrespective of the infants’ age, breastmilk showed similar IgA and IgG antibody levels.

In one woman who had recovered from a COVID-19 infection before the study, IgA levels were already high and consistently high after vaccination. However, the woman’s IgG levels showed a 2-fold increase 10 days after receiving her first Pfizer-BioNTech dose.

An increase in IgG — but not IgA — levels was observed from the donated blood samples of vaccinated women.

Neutralizing antibodies in breastmilk blocks SARS-CoV-2 spike protein binding with ACE2

Neutralizing antibodies detected in breastmilk samples were exposed to four variants of concern: D614G, alpha, beta, and gamma.

Breastmilk from 3 weeks after the second dose successfully blocked ACE2-spike protein binding. Angiotensin-converting enzyme 2 (ACE2) binding from all four SARS-CoV-2 variants was also inhibited. However, the researchers note there was limited ACE2 binding inhibition against the Beta variant.

Neutralizing activity against D614G, Alpha, and Gamma was significantly associated with IgG levels in breastmilk. Neutralization was unaffected by a baby’s age.

SARS-CoV-2 antibodies detected in infant stool samples

Transfer of SARS-CoV-2 antibodies from breastmilk to the infant was confirmed through stool samples. The researchers detected 30% of IgA antibodies and 33% of IgG antibodies targeting the receptor-binding domain in infants as young as 55 days and as old as 11 months.

IgG antibody levels were higher in stool samples when mothers experienced more vaccine side effects.

Increase cytokines levels in breastmilk of vaccinated women who experienced side effects

All breastmilk samples tested positive for the following cytokines: IL-2, IL-6, IL-8, IL-10, IL-13, IL-1β, IFN-γ, and TNF-α.

IFN-γ levels were significantly higher in breastmilk after vaccination than before immunization.

Women who experienced more vaccine side-effects showed higher IFN-γ levels than those who did not. Specifically, women who reported no side effects showed a 2-fold increase after the first dose and a 3-fold increase after the second dose.

In contrast, women who reported side effects showed a 2.5-fold increase in IFN-γ and a 20-fold increase after the second dose.

*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.

Written by

Jocelyn Solis-Moreira

Jocelyn Solis-Moreira graduated with a Bachelor's in Integrative Neuroscience, where she then pursued graduate research looking at the long-term effects of adolescent binge drinking on the brain's neurochemistry in adulthood.