This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Mercedes Yanes Lane

As with many clinical trials, pregnant and lactating women were excluded from initial COVID-19 vaccine trials. Although this is done because of safety concerns, it results in gaps in knowledge to guide vaccine decision-making. In this study in the American Journal of Obstetrics and Gynecology, Dr. Kathryn Gray and colleagues from Harvard Medical School and Brigham and Women’s Hospital in Boston, explore immune responses among pregnant women and women who are breastfeeding following COVID-19 vaccination. Using an observational study model, the authors find that pregnant women and women who are breastfeeding have the same side effects as women who are not pregnant.

This group of researchers in Boston followed pregnant women, women who were breastfeeding, and non-pregnant women after receiving a COVID-19 mRNA vaccine (Pfizer-BioNTech or Moderna/NIH). Blood samples were collected from the women as well as from umbilical cord samples when possible. Breastmilk from lactating women was also collected at several time points after vaccination. Antibody levels were measured in all samples to see if the vaccine was generating an adequate immune response, and if maternal antibodies were being transferred to the child.

A total of 131 women participated in this study: 84 pregnant, 31 breastfeeding, and 16 non-pregnant reproductive-aged women. Of the pregnant vaccine recipients who delivered during the study, umbilical cord blood was collected from 10. Comparable numbers of women received the Pfizer-BioNTech and the Moderna/NIH vaccines.

Reported side effects were mild and included fatigue, muscle aches, rash, headache, and fever. Following the second vaccine dose, there was no measured difference in the reporting of side effects between pregnant and non-pregnant women. The development of antibodies did not differ among women vaccinated in different trimesters of pregnancy and showed a significant rise in all antibody types after the first dose. There was a further rise in IgG (a long-lasting antibody) after the second dose in both pregnant and breastfeeding women.

The levels of antibodies in breastmilk correlate with the antibody levels in maternal blood. There was a high level of IgG found in breastmilk after the second vaccine dose, consistent with what was seen in the blood tests. Maternal IgG antibodies were found to also be capable of crossing the placenta to the child. This specific type of antibody was found in blood from the umbilical cord of all 10 samples collected. An interesting finding was that the transfer of antibodies from mother to child through the placenta increased significantly over time after the second dose.

The authors of this study conclude that COVID-19 mRNA vaccines produce similar levels of antibodies in pregnant and breastfeeding women as in the non-pregnant population. However, this study did not include information on potential risks to the fetus, and thus more information is needed in this area.


Gray KJ, Bordt EA, Atyeo C, Deriso E, Akinwunmi B, Young N, Medina Baez A, Shook LL, Cvrk D, James K, De Guzman R, Brigida S, Diouf K, Goldfarb I, Bebell LM, Yonker LM, Fasano A, Rabi SA, Elovitz MA, Alter G, Edlow AG, COVID-19 vaccine response in pregnant and lactating women: a cohort study. American Journal of Obstetrics and Gynecology (2021). doi:10.1016/j.ajog.2021.03.023.