Vaccines and Immunity that Pass from Mother to Baby
The membranes of the placenta create a barrier that allows some substances to cross to the fetus while keeping other substances out. For example, the placenta does not allow bacteria to enter the fetal bloodstream,1 and can even provide a barrier against some viruses.2 In contrast, the placenta allows water, gases, and nutrients to pass to the fetus. It even transfers some large molecules, like maternal antibodies, to the fetus. These include antibodies against tetanus, diphtheria, and measles,3 but not polio.4 Antibodies are enormous compared to the molecules and chemicals that commonly cross the placenta. Therefore, they need a specific receptor, called the FcRN receptor, to actively move them into the umbilical cord and fetal blood stream.5 While there are five different classes of antibodies, the FcRN receptor moves the smallest type best, the IgG antibody.6 IgG antibodies circulate in the fetus’s blood and provide protection against specific germs and toxins.
The more antibodies that the mother has circulating in her own bloodstream, the more antibodies that will transfer to her baby. That is why doctors recommend getting a vaccine against tetanus, diphtheria, and pertussis (Tdap) between 27 and 30 weeks gestation every time a mother is pregnant, whether or not she already has had the vaccine.8 The vaccine stimulates the mother’s immune system to create more antibodies; therefore, more antibodies transfer to the fetus, helping protect him from whooping cough, caused by the pertussis virus, after birth. Similarly, when mothers get the flu vaccine in the second or third trimester, most of their babies have enough antibodies to avoid getting the flu for 6 months after birth!
Pregnant women appear to be more susceptible to COVID-19, and are at higher risk of serious illness, due to the extra oxygen they need to sustain the fetus.10 Therefore, the CDC recommends that pregnant women get vaccinated against COVID-19. Preliminary studies suggest that the vaccine is safe for pregnant women.11
Since mothers can pass antibodies to their children while they are still pregnant, some researchers have tried to develop vaccines against harmful diseases that can be deadly to newborns, such as group B strep and respiratory syncytial virus (RSV), which can cause colds and pneumonia.12 Currently, a vaccine against Group B strep is in clinical studies with the goal of passing immunity to the young infants,13 but the vaccine against RSV did not produce enough antibodies in non-pregnant volunteers to test in pregnant women.14
Researchers are still investigating the reasons why some antibodies transfer to the fetus efficiently while others do not, although two theories have emerged. First, the FcRN receptor moves more IgG1 antibodies than any other kind. These antibodies identify foreign proteins. This means that the fetus will have better immunity against diseases where the antibodies recognize microbial proteins.15 Second, the placenta selectively transfers antibodies that stimulate the fetus’s own immune system to make natural killer cells.16 Therefore, the placenta is designed to selectively transfer maternal antibodies that will best train the fetus’s immune system to fight future infections itself.
Even after birth, the infant continues to get maternal antibodies through the breastmilk. The most common antibody in breastmilk is IgA, which protects against germs in the respiratory system and gut. The IgA molecules in breastmilk come attached to proteins that protect them from getting digested.17 These IgA molecules attach to the lining of the intestine and protect the infant against ingested bacteria. Infants do not make significant quantities of IgA on their own until several weeks after birth, so the maternal IgA from the breastmilk provides the best defense against germs.18 Maternal antibodies do not pass into the newborn’s bloodstream after birth, but substances in the breastmilk help strengthen a newborn’s own immune responses.19