Pfizer and Moderna vaccines produce robust immune responses in pregnant and nursing women

The findings come from a small study at Beth Israel Deaconess in Boston

Dr. Dan Barouch, head of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, is the senior author of the new study.Lane Turner/Globe Staff

Women who received the Pfizer-BioNTech or Moderna coronavirus vaccine while pregnant or nursing had immune responses as robust as those of other vaccinated women and appeared to pass antibodies to their infants through breast milk and blood, according to a small study at Beth Israel Deaconess Medical Center.

A paper on the findings was published Thursday in the journal JAMA, three weeks after results from a large study of 35,000 US women were reported in the New England Journal of Medicine. It found no increase in miscarriage rates, premature births, and other complications in pregnant women who took either of the two-shot messenger RNA vaccines.

“Together with the safety data that has been published in the New England Journal of Medicine paper, these data support the use of these vaccines in the important populations of pregnant women and lactating women,” said Dr. Dan Barouch, senior author of the new study, of immune responses in 103 women.

Barouch runs the Center for Virology and Vaccine Research at Beth Israel, which licensed technology for the Johnson & Johnson COVID-19 vaccine, the third one cleared for emergency use in the United States. His hospital also is studying how well that vaccine, which requires only one shot, works in pregnant women.

The results of the Beth Israel study are similar to those in another study published in March in the American Journal of Obstetrics and Gynecology by researchers from Massachusetts General Hospital and Brigham and Women’s Hospital. That team studied the same vaccines, but the Beth Israel researchers examined a broader range of immune responses.

Pregnant women with COVID-19 are at significantly higher risk of death and complications for them and their newborns. The growing body of evidence that vaccines are safe and effective may spur more expectant mothers to get one. Pregnant women were excluded from the late-stage clinical trials that prompted federal regulators to authorize the three vaccines.

Kiah Shea, a 32-year-old licensed clinical social worker in Medfield who is due to deliver her second child on Monday, was buoyed by the latest study, which she did not participate in. She got her second Pfizer shot five weeks ago, but, by her own account, is biased: Her father is a pharmaceutical industry veteran who works for Pfizer in Cambridge, although he wasn’t involved in development of the vaccine.

Nonetheless, she carefully weighed the pros and cons of getting the vaccine after her obstetrician said the benefits outweighed the risks.

“Part of me getting it was putting blind trust in science,” said Shea, who has a 20-month-old daughter and is expecting a son. “We don’t know 100 percent. But God forbid I got COVID and was put on a ventilator and had a baby at 28 weeks. Is that a better outcome? Probably not.”

The study at Beth Israel examined the immune responses of 103 women ages 18 to 45 who received a Pfizer or Moderna vaccine from December through March and donated blood and, in some cases, breast milk for the study. Thirty were pregnant, 16 were nursing, and 57 were neither pregnant nor nursing. Some of the expectant and new mothers were patients of obstetrician-gynecologists at the hospital, while others were Beth Israel employees.

The Pfizer and Moderna vaccines stimulated disease-fighting antibodies and T cells in the pregnant and nursing women, just as they did for the women who weren’t pregnant or nursing. That was encouraging, because doctors have long known that pregnancy can suppress the immune system.

“I liken pregnancy to an organ transplant,” said Dr. Ai-ris Collier, an OB-GYN and lead author of the study. “The fetus and placenta have genetic components from both the mom and the father, so there has to be changes in the immune system to prevent outright rejection and to promote fetal growth and survival.”

In addition, antibodies were found in the blood in the umbilical cords that connect infants to mothers, as well as in breast milk. That suggests the antibodies are passed to the babies and may make them immune to the SARS-CoV-2 virus, at least temporarily. Other vaccines cause such temporary passive immunity in infants, including the shot for measles.

The Pfizer and Moderna vaccines also stimulated immune responses against the B.1.1.7 variant, first identified in the United Kingdom, and the B.1.351 variant, first identified in South Africa — in all women in the study, although at reduced levels. This was consistent with earlier studies of how the variants affected the two vaccines.

The vaccines from New York-based Pfizer and Cambridge-based Moderna are the first deployed that rely on synthetic mRNA, an ingenious variation on the natural substance that directs cells to produce proteins.

While traditional vaccines carry a dead or weakened virus into the body to stimulate an immune response, mRNA vaccines use custom-made messenger molecules that tell cells to create a viral protein. The two vaccines instruct cells to create the distinctive spike protein on the coronavirus. Once that happens, the body’s immune system generates antibodies to fend off the disease if the recipient is exposed to the virus.