Whether infants gain protection against the virus that causes COVID-19 through their mother’s breast milk is a key question. Breast milk is a complex biological fluid that provides complete nutrition and a dynamic immune system designed to protect the newborn. This protective capacity is known as passive immunity, where a mother passes her own immune defenses to her child. Understanding the transfer of specific SARS-CoV-2 antibodies through this natural route is important for guiding public health recommendations.
Understanding Antibody Transfer Through Breast Milk
The immune benefit of breast milk is largely due to immunoglobulins, specialized proteins known as antibodies. The dominant antibody found in human milk is secretory Immunoglobulin A (sIgA), which protects the infant’s mucosal surfaces. This sIgA is synthesized by plasma cells that migrate from the mother’s gut and respiratory tract to the mammary glands, ensuring the milk contains antibodies specific to pathogens the mother has recently encountered.
The sIgA molecules are protected from the infant’s digestive environment by a protein shield called the secretory component. This coating allows the antibodies to survive stomach acid and digestive enzymes as they travel through the infant’s gastrointestinal and respiratory tracts. Instead of being absorbed into the baby’s bloodstream, sIgA primarily coats the mucosal lining of the gut and airways. This localized action neutralizes pathogens, preventing them from attaching to the baby’s cells and causing infection.
While sIgA provides surface-level defense, Immunoglobulin G (IgG) is the primary type transferred to the fetus across the placenta before birth. This provides the newborn with systemic immunity that circulates in the blood for several months. Breast milk contains small amounts of IgG, but the majority of antibody protection provided through nursing is the mucosal defense conferred by sIgA.
Direct Evidence of SARS-CoV-2 Antibodies
Research confirms that specific antibodies targeting SARS-CoV-2 are present in the breast milk of mothers exposed to the virus or who received a vaccine. These antibodies are directed against the virus’s Spike protein, the structure the virus uses to enter human cells. The presence of these specific antibodies indicates that the mother’s immune system has generated a targeted response transferred to the infant.
Both IgA and IgG antibodies specific to SARS-CoV-2 have been detected in human milk samples following maternal exposure. These antibodies retain their neutralizing capacity, meaning they are functional and can disable the virus. This neutralizing effect occurs when the antibodies bind to the Spike protein, blocking the virus from infecting cells in the baby’s mucosal tissues.
The primary function of these milk-borne antibodies is to provide “immune exclusion” at the virus’s entry points: the nose, throat, and gut. While this mucosal protection is a localized defense, it may play a role in preventing initial infection or reducing illness severity. Furthermore, studies consistently find that the infectious SARS-CoV-2 virus is rarely present in breast milk.
Maternal Status: Vaccination Versus Natural Infection
The type and quantity of SARS-CoV-2 antibodies transferred into breast milk differ based on whether the mother acquired immunity through vaccination or natural infection. Natural infection often results in a higher concentration of IgA antibodies in the milk. This IgA-dominant response aligns with the body’s strategy for fighting infections that enter through a mucosal surface.
In contrast, mothers who received an mRNA vaccine show a more robust and dominant IgG antibody response in their breast milk. Vaccination elicits a strong systemic immune reaction, leading to high levels of IgG circulating in the blood, which is then transferred into the milk. Both IgA and IgG antibodies generated through either route neutralize the SARS-CoV-2 virus, offering a protective benefit.
The persistence of these antibodies also varies. Antibodies generated from vaccination, particularly after a booster dose, remain detectable in breast milk for a longer duration, sometimes up to eight months. While antibodies from a natural infection are protective, their levels may decline more rapidly compared to vaccine-induced antibodies. The specific type of vaccine used, such as mRNA or viral vector, does not significantly change the antibody profile in the milk.
Practical Implications for Infant Protection
The presence of functional SARS-CoV-2 antibodies in breast milk provides passive immunity that helps protect the nursing infant. This protection is valuable for babies too young to receive their own COVID-19 vaccine, typically those under six months of age. The milk-borne antibodies act as a temporary shield, especially in the baby’s respiratory and digestive tracts, where the virus attempts to enter the body.
This passive immunity from breast milk is not equivalent to the active, systemic protection provided by infant vaccination. The antibodies guard the baby’s mucosal surfaces but cannot fully prevent a systemic infection from taking hold. However, this protection may lessen the severity of the illness if the infant does become infected, similar to how maternal antibodies protect against other childhood diseases.
For mothers who have COVID-19 or are concerned about exposure, major pediatric and health organizations consistently recommend continuing to breastfeed. The scientific consensus is that the benefits of breastfeeding outweigh the minimal risk of transmission, as the virus is rarely transmitted through the milk itself. Breastfeeding remains a safe and recommended practice, offering a valuable layer of defense for the infant during the pandemic and beyond.