April 20, 2021 -- As the U.S. races to vaccinate millions of people against the coronavirus, pregnant women face the extra challenge of not knowing whether the vaccines are safe for them or their unborn babies.

None of the recent COVID-19 vaccine trials, including those for Pfizer, Moderna, and Johnson & Johnson, enrolled pregnant or breastfeeding women because they consider them a high-risk group.

That was despite the Society for Maternal-Fetal Medicine and the American College of Obstetricians and Gynecologists asking that pregnant and breastfeeding women be included in trials. The FDA even included pregnant women in the COVID-19 vaccine emergency use authorization (EUA) due to their higher risk of having a more severe disease.

Despite that lack of clinical trial data, more and more smaller studies are suggesting that the vaccines are safe for both mother and child.

Pfizer is now studying its two-dose vaccine in 4,000 pregnant and breastfeeding women to see how safe, tolerated, and robust their immune response is. Researchers will also look at how safe the vaccine is for infants and whether mothers pass along antibodies to children. But the preliminary results won’t be available until the end of the year, a Pfizer spokesperson says.

Without that information, pregnant women are less likely to get vaccinated, according to a large international survey. Less than 45% of pregnant women in the U.S. said they intended to get vaccinated even when they were told the vaccine was safe and 90% effective. That figure rises to 52% of pregnant women in 16 countries including the U.S., compared to 74% of nonpregnant women willing to be vaccinated. The findings were published online March 1 in the European Journal of Epidemiology.

The vaccine-hesitant pregnant women in the international study were most concerned that the COVID-19 vaccine could harm their developing fetuses, a worry related to the lack of clinical evidence in pregnant women, says lead researcher Julia Wu, doctor of science and an epidemiologist at the Harvard T.H. Chan School of Public Health’s Human Immunomics Initiative in Boston.

The information vacuum also increases the chances that “people will fall victim to misinformation campaigns like the one on social media that claims that the COVID-19 vaccine causes infertility,” Wu says. This unfounded claim has deterred some women of childbearing age from getting the vaccine.

Deciding to Get Vaccinated

Front-line health care professionals were in the first group eligible to receive the vaccine last December. “All of us who were pregnant … had to decide whether to wait for the data, because we don’t know what the risks are, or go ahead and get it [the vaccine]. We had been dealing with the pandemic for months and were afraid of being exposed to the virus and infecting family members,” says Jacqueline Parchem, MD, a maternal-fetal medicine specialist at McGovern Medical School at the University of Texas Health Science Center in Houston.

Given the lack of safety data, the CDC guidance to pregnant women has been to consult with their doctors and that it’s a personal choice. The CDC’s latest vaccine guidance says “there is no evidence that antibodies formed from COVID-19 vaccination cause any problem with pregnancy, including the development of the placenta.”

The CDC is monitoring vaccinated people through its v-safe program and reported April 12 that more than 86,000 v-safe participants said they were pregnant when they were vaccinated.

Health care workers who were nursing their infants when they were eligible for the vaccine faced a similar dilemma as pregnant women -- they lacked the data on them to make a truly informed decision.

“I was nervous about the vaccine side effects for myself and whether my son Bennett, who was about a year old, would experience any of these himself,” says Christa Carrig, a labor and delivery nurse at Massachusetts General Hospital in Boston, who was breastfeeding at the time.

She and Parchem know that pregnant women with COVID-19 are more likely to have severe illness and complications such as high blood pressure and preterm delivery. “Pregnancy takes a toll on the body. When a woman gets COVID-19 and that insult is added, women who were otherwise young and healthy get much sicker than you would expect,” says Carrig.

“As a high-risk pregnancy specialist, I know that with COVID, that babies don’t do well when moms are sick,” says Parchem.

Pregnant women accounted for more than 84,629 cases of COVID-19 and 95 deaths in the U.S. between Jan. 22 last year and April 12 this year, according to the CDC COVID data tracker.

Parchem and Carrig decided to get vaccinated because of their high risk of exposure to COVID-19 at work. After the second dose, Carrig reported chills but Bennett had no side effects from breastfeeding. Parchem, who delivered a healthy baby boy in February, reported no side effects other than a sore arm.

“There’s also a psychological benefit to returning to some sense of normalcy,” says Parchem. “My mother was finally able to visit us to see the new baby after we were all vaccinated. This was the first visit in more than a year.”

New Study Results

Carrig was one of 131 vaccinated hospital workers in the Boston area who took part in the first study to profile the immune response in pregnant and breastfeeding women and compare it to both nonpregnant and pregnant women who had COVID-19.

The study was not designed to evaluate the safety of the vaccines or whether they prevent COVID-19 illness and hospitalizations. That is the role of the large vaccine trials, say the authors.

The participants were between the ages of 18 and 45 and received both doses of either Pfizer or Moderna vaccines during one of their trimesters. They provided blood and/or breast milk samples after each vaccine dose, 2-6 weeks after the last dose, and at delivery for the 10 who gave birth during the study.

The vaccines produced a similar strong antibody response among the pregnant/breastfeeding women and nonpregnant women. Their antibody levels were much higher than those found in the pregnant women who had COVID-19, the researchers reported in the March 25 issue of the American Journal of Obstetrics and Gynecology.

“This is important because a lot of people tend to think once they’ve had COVID-19, they are protected from the virus. This finding suggests that the vaccines produce a stronger antibody response than the infection itself, and this might be important for long-lasting protection against COVID-19,” says Parchem.

The study also addressed whether newborns benefit from the antibodies produced by their mothers. “In the 10 women who delivered, we detected antibodies in their umbilical cords and breast milk,” says Andrea Edlow, MD, lead researcher and a maternal-fetal medicine specialist at Massachusetts General Hospital in Boston.

Newborns are particularly vulnerable to respiratory infections because they have small airways and their immune systems are under-developed. These infections can be lethal early in life.

“The public health strategy is to vaccinate mothers against respiratory viruses, bacteria, and parasites that neonates up to 6 months are exposed to. Influenza and pertussis (whooping cough) are two examples of vaccines that we give mothers that we know transfer [antibodies] across the umbilical cord,” says Edlow.

But this “passive transfer immunity” is different from active immunity, when the body produces its own antibody immune response, she explains.

A different study, also published last month, confirmed that antibodies were transferred from 27 vaccinated pregnant mothers to their infants when they delivered. A new finding was that the women who were vaccinated with both doses and earlier in their third semester passed on more antibodies than the women who were vaccinated later or with only one dose.

Impact of the Studies

The Society for Maternal-Fetal Medicine updated its guidance on counseling pregnant and lactating patients about the COVID-19 vaccines to include Edlow’s study.

“We were struck by how much pregnant and breastfeeding women want to participate in research and to help others in the same situation make decisions. I hope this will be an example to drug companies doing research on new vaccines in the future -- that they should not be left behind and can make decisions themselves whether to participate after weighing the risks and benefits,” says Edlow.

She continues to enroll more vaccinated women in her study in the Boston area, including non-health care workers who have asked to take part.

"It was worth getting vaccinated and participating in the study. I know that I have antibodies and it worked and that I passed them on to Bennett. Also, I know that all the information is available for other women who are questioning whether to get vaccinated or not,” says Carrig.

Parchem is also taking part in the CDC’s v-safe pregnancy registry, which is collecting health and safety data on vaccinated pregnant women.

Before she was vaccinated, Parchem says, “my advice was very measured because we lacked data either saying that it definitely works or showing that it was unsafe. Now that we have this data supporting the benefits, I feel more confident in recommending the vaccines.”

Show Sources

Julia Wu, ScD, lead researcher and epidemiologist, Harvard T.H. Chan School of Public Health’s Human Immunomics Initiative, Boston.

Jacqueline Parchem, MD, maternal-fetal medicine specialist, McGovern Medical School, University of Texas Health Science Center, Houston.

Christa Carrig, registered nurse; labor and delivery nurse, Massachusetts General Hospital, Boston.

Andrea Edlow, MD, lead researcher and a maternal-fetal medicine specialist, Massachusetts General Hospital, Boston.

CDC COVID Data Tracker: “Data on COVID-19 during pregnancy: severity of maternal illness.”

CDC: “V-safe COVID-19 Vaccine Pregnancy Registry,” “Information about COVID-19 Vaccines for People who Are Pregnant or Breastfeeding.”

American College of Obstetricians and Gynecologists: “Vaccinating Pregnant and Lactating Patients Against COVID-19.”

The Hill: “Vaccine research must include pregnant women during COVID and beyond.”

Society for Maternal-Fetal Medicine: “SMFM: Provider Considerations for Engaging in COVID-19 Vaccine Counseling with Pregnant and Lactating Patients 4.1.2021 (last published 3.3.21).”

News release, Massachusetts General Hospital: “Pregnant and lactating women show robust immune response to COVID vaccines, pass antibodies to newborns.”

American Journal of Obstetrics and Gynecology: “Cord Blood Antibodies following Maternal COVID-19 Vaccination During Pregnancy,” “COVID-19 vaccine response in pregnant and lactating women: a cohort study.”

European Journal of Epidemiology: “COVID-19 vaccine acceptance among pregnant women and mothers of young children: results of a survey in 16 countries.”

News release, Pfizer: “Pfizer and BioNTech commence global clinical trial to evaluate covid-19 vaccine in pregnant women.”

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