When a pregnant individual contracts COVID-19, their immune system produces antibodies that can be shared with the developing fetus. This process is known as passive immunity, a temporary form of protection where ready-made antibodies are transferred from mother to child. These maternal antibodies offer the newborn a degree of defense against the virus immediately after birth, when their own immune system is still immature. This is not a guarantee of complete immunity, but a protective shield that can significantly lower the risk of severe illness, hospitalization, or death in the early months of life.
Passive Immunity Transfer During Pregnancy
The primary mechanism for transferring immunity before birth is through the placenta, the organ responsible for nutrient and oxygen exchange between the mother and the fetus. This specialized transfer pathway specifically uses a type of antibody called Immunoglobulin G, or IgG, which is the smallest and most abundant antibody in the bloodstream. IgG antibodies are actively transported from the maternal circulation across the placental barrier and into the fetal bloodstream, providing systemic protection throughout the baby’s body.
The efficiency of this transfer depends on the timing of the maternal infection during pregnancy. The passage of IgG is most effective during the later stages of gestation, particularly the second and third trimesters, as placental transport mechanisms mature. If the mother’s infection occurs more than 60 days before delivery, the fetus typically receives a higher concentration of protective antibodies. An acute infection very close to delivery can sometimes impair the efficiency of this transplacental transfer.
The amount of IgG transferred is often directly proportional to the mother’s own antibody levels at the time of delivery. A robust maternal immune response, whether from infection or vaccination, is necessary to ensure a high concentration of protective antibodies reaches the fetus. Once in the baby’s circulation, these maternal IgG antibodies function as a temporary defense force until the infant’s own immune system can fully develop and respond to pathogens.
How Long Does This Protection Last?
The protection a baby receives from maternal antibodies is temporary because the infant’s immune system did not produce them and lacks the “memory” to replenish them. This passive immunity wanes over time as the antibodies naturally break down in the infant’s circulation, a process known as antibody catabolism. The half-life of these maternally-derived antibodies dictates how long they remain at protective levels.
Current research suggests that this passive protection may persist in the infant for an estimated three to six months following birth. Antibody levels begin to steadily decrease from the time of delivery, with the rate of decline depending on the initial concentration received. For instance, infants who received a very high level of IgG at birth tend to maintain detectable levels for a longer duration.
The practical benefit of this temporary shield is the reduction in the severity of illness. While the antibodies may not prevent the baby from contracting the virus entirely, they are effective at neutralizing the pathogen enough to prevent serious outcomes. This protection is especially valuable during the first half-year of life, when infants are most vulnerable and before they are eligible for their own COVID-19 vaccination.
Postnatal Immunity Through Breast Milk
Beyond the prenatal transfer of IgG, the mother can provide a secondary layer of passive immunity after birth through breastfeeding. Breast milk, particularly the early milk known as colostrum, contains a different type of antibody that provides localized protection. This is primarily Immunoglobulin A (IgA), which is the dominant antibody found in human milk.
Unlike IgG antibodies that circulate systemically, IgA antibodies function mainly on mucosal surfaces, such as the lining of the infant’s respiratory and gastrointestinal tracts. This local defense works by binding to pathogens, like SARS-CoV-2, preventing them from attaching to and entering the cells of the mucous membranes. This minimizes the risk of infection at the common entry points for the virus.
The IgA in breast milk is produced via a specialized process called the entero-mammary pathway, which allows the mother to produce antibodies specific to the pathogens she has encountered. This mechanism ensures that the baby receives ongoing, tailored immune support for as long as they are breastfed. This continuous supply of IgA acts as a surface barrier, complementing the systemic protection initially provided by the transferred IgG.
Protection from Infection Versus Vaccination
The quality and quantity of antibodies transferred to the infant differ significantly depending on whether the mother developed antibodies from a natural COVID-19 infection or from a COVID-19 vaccine. Maternal vaccination, particularly with mRNA vaccines, generates a more robust and consistent level of neutralizing antibodies in the mother. This high concentration in the mother translates into a more substantial transfer of antibodies to the fetus via the placenta.
Vaccination typically produces higher antibody levels in both the maternal blood and the umbilical cord blood at delivery compared to antibodies generated by a natural infection, especially if the infection was mild. The sustained presence of these antibodies in the infant is also greater following maternal vaccination. For example, six months after birth, a higher percentage of infants born to vaccinated mothers still had detectable IgG antibodies compared to infants born to mothers who had been infected but not vaccinated.
A further consideration is the risk profile associated with each method of gaining immunity. While both infection and vaccination can confer passive immunity, vaccination provides the benefit of antibody transfer without exposing the mother and fetus to the potential health risks of the SARS-CoV-2 disease itself. Therefore, vaccination is considered the optimal strategy for maximizing the protective antibody transfer to the infant while safeguarding the health of the pregnant individual.