Research shows that COVID-19 antibodies can be transferred across the placenta from mom to baby. This suggests newborns of mothers who have been previously infected with the COVID-19 virus may have some protection against COVID-19 at birth. 

One study tested blood samples of 1,471 mothers and their newborns for COVID-19 antibodies. In the study, 87% of mothers transferred antibodies to their infants. 

The more antibodies a mother had in the study, the more they passed to their babies. The transfer of antibodies happened with mothers whose former COVID-19 infections were asymptomatic, mild, moderate, and severe.

Antibodies were not passed to all newborns in the study, and more research is needed to understand why. Continuing research is important because the results are mixed. A second, preliminary, study found a much lower rate of antibody transfer from mothers to infants —around 25% as opposed to 87%. Though this study had a much smaller sample size of 32 women-newborn pairs, the large differences in the data suggests the need for more research.

Several studies also suggest that women who received an mRNA vaccine—such as Moderna or Pfizer—during pregnancy were able to transfer antibodies to infants. This suggests that mothers can pass antibodies to their infants after vaccination.

Antibodies in mothers’ breastmilk have also been noted in some studies. One preprint study published in March 2021 studied more than 130 vaccinated women. Eighty-four were pregnant, 10 delivered during the study, and 31 were lactating. Among the 10 women who delivered their babies during the study, all had detectable antibodies in their blood, and nearly all of the lactating women had antibodies in their breastmilk.

Researchers for another study found that women who had previously tested positive for COVID-19 or were likely infected with the virus and recovered had COVID-19-specific antibodies. These results further confirm the presence of antibodies in breastmilk, which can then be passed to infants who are breastfeeding.

Research suggests that the vaccine is of great benefit to mothers and children. This is a crucial finding given that mothers and children are particularly vulnerable to respiratory diseases such as COVID-19. More research is needed to understand exact rates of transfer across different populations and with different vaccines.

During the last 3 months of pregnancy, antibodies from a pregnant individual are typically passed to the unborn baby through the placenta. This type of immunity transferred to an infant is called “passive immunity,” because the baby has been given antibodies from the mother rather than making them on their own. The level and specific type of antibodies transferred to the baby depends on the mother's level of immunity, how long it's been since the mom was infected, and the type of virus. 

Breastmilk also contains antibodies, which means that babies who are breastfed have passive immunity for a longer period of time. A thick yellowish milk known as colostrum that mothers produce the first few days following birth is particularly rich in antibodies. Premature babies are at higher risk of developing illness because their immune systems aren't as strong and they haven't had as many antibodies passed to them.

Given that passive immunity is expected across the placenta and through breastmilk, it isn’t surprising that we are seeing COVID-19 antibodies passed from mothers to infants. Given that it’s such a new virus, it requires careful studying to understand how, and at what rate, immunity is transferred from mother to infant following natural infection, and especially following vaccination, as vaccines continue to roll out globally.