Answering Questions with Research

“With outbreaks of new diseases like COVID-19, I worry about possible effects on pregnancy and whether pregnancy might worsen symptoms,” Edlow said. “When the COVID-19 pandemic began, no one knew how the virus affects pregnancy.”

It turns out that Edlow was right to worry about the effects of SARS-CoV-2. Although most pregnant people do fine if they get COVID-19, studies showed that pregnancy increases the risk of severe disease and death.²

When the U.S. Food and Drug Administration authorized the first COVID-19 vaccines in December 2020, many people wondered about the vaccines’ effects during pregnancy.

“No special pregnancy concerns had shown up in animal tests of the COVID-19 mRNA vaccines,” Edlow said. “But we didn’t have any ‘gold-standard’ evidence from human studies on the use of vaccines during pregnancy.”

Edlow sought to get the information pregnant people need by checking antibody responses to the Pfizer and Moderna mRNA vaccines in pregnant, nonpregnant, and lactating women. For women who gave birth during the study, the researchers also looked for antibodies in samples of breastmilk and in umbilical cord blood.

A total of 131 women (84 pregnant, 31 lactating, and 16 nonpregnant women) who were planning to get vaccinated volunteered for the study. (Vaccinations were not given as part of the research.) Blood samples from 37 unvaccinated women who had COVID-19 infections during pregnancy were also tested. The study included a questionnaire asking about side effects of vaccination.

Vaccines Are Safe and Effective for Pregnant People

Edlow’s study results showed vaccination is safe during pregnancy.

“Vaccination is not associated with miscarriage or preterm birth or infertility,” Edlow explained. “Side effects are minor and similar to those reported by people who aren’t pregnant. Possible vaccine side effects should not deter women from getting vaccinated.”

Women who were vaccinated while pregnant developed enough antibodies to protect them from infection — that is, the vaccine worked.

“We learned that vaccination during pregnancy stimulates strong immune responses in women,” Edlow explained. “This is important new knowledge since the vaccines were not originally tested in pregnant people.”

The research team also compared antibody levels in women who were vaccinated while pregnant and antibody levels in unvaccinated pregnant women who got COVID-19. The women who had COVID-19 had weaker antibody responses than women who were vaccinated.

Vaccinating pregnant people may also benefit their babies.

“Antibodies go from the mother’s bloodstream to the placenta and then travel through the umbilical cord to the fetus,” Edlow said. “The antibodies may protect the baby for six months or more after birth.”

In addition, the study showed that the mother’s antibodies make their way into her breastmilk, likely protecting infants from COVID-19 for at least a few days after they consume the milk.

In late September 2021, the Centers for Disease Control and Prevention (CDC) issued an urgent health advisory to increase vaccination among pregnant people and people who may become pregnant. At that time, less than one-third of pregnant women had been vaccinated against COVID-19, despite the benefits of vaccination far outweighing known or potential risks.

Making Decisions About Vaccination: Two Stories

Getting vaccinated as soon as possible is the safest choice for most pregnant people and their babies. The Centers for Disease Control and Prevention now encourages all pregnant people to get vaccinated to protect themselves from COVID-19.

In July 2021, two major professional societies dedicated to pregnancy care strongly recommended that pregnant people be vaccinated to protect them from COVID-19.³ The groups relied on data from several studies, including Edlow’s, as well as information from more than 139,000 people who reported being pregnant when they were vaccinated.⁴

However, many pregnant people — including Clark — had to decide about vaccination before any data were available. Clark explained her reasons for getting Moderna vaccine doses in early 2021.

“I’d already gotten the flu vaccine while I was pregnant and didn’t have any reaction,” Clark said. “My doctor thought that the risks of the mRNA vaccines would be low, because few problems had been seen among the many people who had been vaccinated. As a first-line care provider, I was much more concerned about getting COVID-19 than the vaccine,” Clark recalled.

Raina Gay Leahy, Ph.D., also participated in Edlow’s study. Leahy is an associate director for clinical operations at a pharmaceutical company, managing clinical trials for treatments for a rare condition called hereditary transthyretin amyloidosis. Leahy got the news that she was pregnant with her third child during the summer of 2020. She opted to get vaccinated as soon as she was eligible in early 2021.

“SARS-CoV-2 is a weird virus that has unexpected health effects. Based on my scientific background and the information I’d read about the vaccines, I felt confident about their safety, even for pregnant people,” Leahy said. “I’d rather put up with some flu-like side effects and a sore arm from vaccination than face the risk of lung infections, blood-clotting problems, and pregnancy complications that COVID-19 can cause.”

Leahy hopes that Edlow’s study results will reduce concerns about vaccination during pregnancy.

“Having scientific evidence that the vaccines work during pregnancy and knowing that a mother’s antibodies may protect her newborn will encourage more pregnant women to get vaccinated,” Leahy said.