Does a breastfeeding mother who receives Covid (Coronavirus) and flu (influenza) vaccines pass on antibodies to her baby through breast milk, providing passive immunity?

Passive Immunity Through Breastfeeding After COVID-19 and Influenza Vaccination

Yes, breastfeeding mothers who receive COVID-19 and influenza vaccines do pass protective antibodies to their babies through breast milk, providing passive immunity. This antibody transfer has been clearly demonstrated for both vaccines and offers meaningful protection to infants during their vulnerable early months of life.

Evidence for Antibody Transfer

COVID-19 Vaccination

• Breastfeeding mothers vaccinated against COVID-19 produce significant levels of SARS-CoV-2-specific IgA and IgG antibodies in their breast milk, with detection occurring after both the first and second vaccine doses 1.
• These antibodies demonstrate functional neutralizing capacity against SARS-CoV-2, including protection against variants like Omicron, though with somewhat reduced effectiveness against newer variants 2.
• The antibody levels in breast milk show moderate positive correlation with maternal serum levels (r = 0.73), though serum concentrations are generally higher 2.
• Vaccine-related mRNA has not been detected in breast milk, confirming safety for the infant while antibodies are successfully transferred 3, 4.
• Mothers who had COVID-19 infection during pregnancy and then received vaccination during lactation showed significantly higher antibody levels in breast milk (35.15 AU/ml) compared to those with infection alone (1.26 AU/ml) or vaccination alone (4.52 AU/ml) 2.

Influenza Vaccination

• Breastfeeding mothers vaccinated against influenza produce breast milk containing higher levels of influenza-specific IgA antibodies 4.
• These antibodies remain elevated for up to 6 months after birth in women vaccinated during pregnancy 4.
• Infants born to mothers vaccinated against influenza during pregnancy have a 72% risk reduction for laboratory-confirmed influenza hospitalization in the first few months of life 4.
• Greater exclusivity of breastfeeding in the first 6 months decreases episodes of respiratory illness with fever in infants of vaccinated mothers 4.
• Breastfeeding also activates innate antiviral mechanisms in infants, specifically type-1 interferons, providing additional protection beyond passive antibody transfer 4.

Dual Protection Mechanism

Maternal vaccination provides two complementary pathways of protection 4:

• Transplacental transfer during pregnancy: Primarily IgG antibodies cross the placenta, especially when vaccination occurs in the third trimester 3.
• Breast milk transfer after birth: Primarily IgA antibodies (the mucosal immune system's main defender) pass through breast milk during lactation 3, 4.

This dual mechanism means that vaccination during pregnancy provides initial protection through placental transfer, which is then sustained and enhanced through ongoing antibody transfer via breastfeeding 4.

Clinical Implications and Safety

Safety Profile

• All major medical organizations agree that COVID-19 and influenza vaccination poses minimal to no risk to breastfed infants, including the Academy of Breastfeeding Medicine and the American College of Obstetricians and Gynecologists 3, 4.
• No COVID-19-related deaths were reported among 1,346 newborns in studies examining breastfeeding from vaccinated or infected mothers 5.
• Breastfeeding should not be discontinued for mothers receiving COVID-19 or influenza vaccines 3.

Vaccine Technology Considerations

For COVID-19 vaccines, different platforms show varying levels of antibody transfer 6:

• Pfizer-BioNTech (mRNA): Greatest neutralizing effect in breast milk
• Johnson & Johnson (adenovirus vector): Moderate neutralizing effect
• CanSino (adenovirus vector): Least neutralizing effect

Only mRNA vaccines (Pfizer-BioNTech and Moderna) are specifically approved for pregnant and lactating women in most guidelines 3.

Optimal Timing Recommendations

For Maximum Infant Protection

• Influenza vaccination should be administered as soon as the seasonal vaccine becomes available, ideally by the end of October 4.
• Pregnant women in their third trimester can receive influenza vaccination during July and August to maximize antibody transfer before delivery 4.
• Both COVID-19 and influenza vaccines can be administered simultaneously at different anatomic sites 4.

Booster Considerations

• Booster vaccinations are particularly important for mothers who had COVID-19 infection during pregnancy, as a single vaccine dose after infection dramatically increases breast milk antibody levels 2.
• This strategy optimizes protection for both mother and newborn 2.

Common Pitfalls to Avoid

• Do not delay or discontinue breastfeeding due to maternal vaccination—the benefits far outweigh any theoretical risks 3.
• Do not postpone the infant's scheduled vaccinations (such as 4-month vaccines) due to maternal vaccination; there is no evidence of interference 4.
• Do not assume that natural infection alone provides optimal protection—vaccination after infection significantly enhances antibody transfer through breast milk 2.
• Do not group pregnancy and lactation together when considering vaccine safety, as these are physiologically distinct states with different risk profiles 3.