Are the SARS-CoV-2 Vaccines Safe for Pregnant and Lactating People?

by Gertrud U. Rey

None of the clinical trials for SARS-CoV-2 vaccines included pregnant or lactating subjects. Pregnant people in particular are a historically difficult group to study in the context of drug safety because of concerns of liability over potential adverse effects of a new product on a fetus. This lack of safety data further complicates treatment of SARS-CoV-2 infection in pregnant people, who already have an increased risk for premature birth and miscarriage if they become infected. Based on these risk factors, the currently available data, and interim recommendations by the Advisory Committee on Immunization Practices for people over the age of 16, the American College of Obstetricians and Gynecologists recommends that pregnant and lactating people should have access to SARS-CoV-2 vaccines. Fortunately, many pregnant and lactating people have opted to be vaccinated, and the data on the safety and efficacy of vaccination in these two cohorts are beginning to emerge.

A recent publication in the American Journal of Obstetrics and Gynecology describes the safety and immunogenicity of SARS-CoV-2 vaccines in pregnant and lactating women compared to non-pregnant controls and women who had been naturally infected with SARS-CoV-2 during pregnancy. The researchers collected blood and breast milk from 131 women of reproductive age, including 84 pregnant, 31 lactating, and 16 non-pregnant participants who had received two doses of either the Pfizer/BioNTech or Moderna mRNA COVID-19 vaccine; and analyzed the samples for the presence of IgG and IgA antibodies. IgG antibodies are mostly present in the blood and provide the majority of antibody-based immunity against invading pathogens. IgA antibodies are predominantly found in mucus membranes and the fluids secreted by these membranes, including saliva, tears, and breast milk. IgA antibodies are particularly suitable for protecting against pathogens like SARS-CoV-2, which invade the body through these membranes.

Samples were collected at the time of the first and second vaccination, 2 to 6 weeks after the second dose, and at delivery. The researchers also collected umbilical cord blood from 10 infants born during the study period.

The main findings were as follows. Side effects resulting from vaccination were rare and occurred with similar frequency in all participants. These findings are consistent with post-vaccination data tracked by the CDC via the V-safe application, suggesting that post-vaccination reactions in pregnant people are not significantly different from those in non-pregnant people.

Pregnant, lactating, and non-pregnant vaccine recipients produced significantly higher levels of SARS-CoV-2-specific IgG than pregnant women who had been naturally infected. Interestingly, the second vaccine dose led to increased concentrations of coronavirus-specific IgG but not IgA, in both maternal blood and breast milk. The authors speculate that this lack of IgA boosting may have to do with the intramuscular route of immunization, which usually does not induce very high levels of serum IgA. The role of IgA in the serum is mostly secondary to IgG, in that IgA mediates elimination of pathogens that have breached the mucosal surface. In contrast, natural infection induces higher levels of mucosal (IgA) antibodies because SARS-CoV-2 enters the body through the mucosal surfaces. Nevertheless, studies have shown that IgG in breast milk is critical for protecting newborns from other pathogens like HIV, RSV, and influenza. It is possible that this type of antibody is equally important for protecting infants from SARS-CoV-2.

It was previously unclear whether vaccine-induced SARS-CoV-2 antibodies are transferred across the placenta to the fetus. However, the authors also found vaccine-induced IgG in cord blood, suggesting that these antibodies do cross the placenta, as has been previously observed after vaccination of pregnant people against influenza virus, Bordetella pertussis, and other pathogens. Whether these antibodies protect the baby from SARS-CoV-2 infection after birth, remains to be determined.

This study provides the first set of clinical data suggesting that vaccination of pregnant and lactating people against SARS-CoV-2 is both safe and beneficial. Some people have expressed concern over the possible incidence of fever following the second vaccine dose in pregnant people; however, the potential risk to the fetus from a 24-hour fever must be weighed against the possibility of chronic, severe COVID-19 in the mother. Accumulating data suggest that the benefits of vaccination during pregnancy and lactation outweigh the risks of adverse effects to the fetus or infant and align with the abundance of clinical data showing the beneficial effects of vaccinating these groups of people against influenza virus. Analyses of cord blood from infants whose mothers were naturally infected with SARS-CoV-2 during pregnancy indicate that the potential for transfer of antibodies is greater early during gestation, suggesting that immunization during the earlier stages of pregnancy might be preferable. However, more studies are needed to assess the optimal timing of maternal vaccination to achieve enhanced neonatal immunity. In the meantime, these preliminary data allow pregnant and lactating people to make more informed decisions, and also aid physicians in providing evidence-based recommendations.