Alice Proujansky for The New York Times

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By Gil Mor

Dr. Mor, a reproductive immunologist, leads a research lab at Wayne State University that studies the immune system during pregnancy and the impact of pathogens.

More than 60 years ago, the Nobel Prize-winning biologist Peter Medawar posed what has become known as the immunological paradox of pregnancy.

The fetus, Dr. Medawar argued, is like a semiforeign transplant because half of its genes come from the father. Therefore, the mother’s immune system and the fetus must be locked in conflict. One of Dr. Medawar’s theories for why the mother’s body does not reject the pregnancy was that the maternal immune system is inhibited. As a result, the concept of pregnancy as an immune-suppressed condition was introduced to scientists, and it has influenced thinking about pregnancy among doctors and the public ever since.

But subsequent research, including my own, has led to the development of a different, more optimistic perspective on how the pregnant woman’s immune system and the fetus interact. Rather than threatening the fetus, the maternal immune system plays a critical role in the success of pregnancy, particularly in its early stages. Yes, a mother’s immune system is changed during pregnancy. But it is stronger in many ways, not weaker. The old science has led to bad advice, especially during pandemics.

As a former colleague and I predicted a decade ago, the generalization that pregnant women are immune-suppressed and therefore at a higher risk for infections not only is misleading but also “prevents the determination of adequate guidelines for treating pregnant women during pandemics.”

Dr. Medawar’s paradox has, unintentionally, led to another paradox: A medical system that typically devotes much care and attention to pregnant women and their fetuses has instead neglected them or given them infrequently updated and conflicting advice on how to protect themselves. This problem exists for Covid-19 and has held true for pregnant women navigating other infectious disease outbreaks, like Ebola in West Africa.

OPINION CONVERSATION

Questions surrounding the Covid-19 vaccine and its rollout.

Should pregnant women get the vaccine?
Gil Mor, a reproductive immunologist, explains why old science has led to conflicting advice.

As more vaccine mandates arrive, how will we handle verification?
Tom Frieden, a former director of the C.D.C., describes how a safe and secure system could work.

Are vaccine mandates a problem for civil liberties?
Two writers from the A.C.L.U. argue that actually, it’s quite the opposite.

How many people have died because of undervaccination?
Comparing different areas of the U.S. suggests there have been many preventable deaths.

It took too long for pregnant women to be included in clinical trials for the Covid-19 vaccines and for public health agencies in the United States and other countries to make recommendations. Without that guidance, physicians did not have the information needed to advise their pregnant patients, and many pregnant women may have defaulted to a view that a vaccine could be unsafe for them.

Now the data is in: The risks of Covid-19 far outweigh the risks to pregnant women of getting vaccinated, and the Centers for Disease Control and Prevention is pleading with them to get their shots. Yet according to C.D.C. data, only 31 percent of pregnant people in the United States have been vaccinated against Covid-19. Even though the absolute risk is low, compared to nonpregnant symptomatic people with Covid-19, those who are pregnant and symptomatic have a more than twofold increased risk of being admitted into an intensive care unit, the C.D.C. reports, and a 70 percent increased risk of death from the disease. Pregnant women with Covid are also at a higher risk of complications like preterm birth or stillbirth or their newborns being admitted to an intensive care unit compared to pregnant people without Covid-19.

Some viral infections, such as by the Zika virus, are a unique risk during pregnancy. It is not because pregnant women’s immune systems are weak but because of the possibility of the virus reaching the fetus — which is not the case for every virus. It’s also possible that some of the ways a pregnant woman’s immune system responds to a viral infection could affect the fetus and its development. Answering these questions should be a priority in every epidemic.

To better understand the impact of Covid-19 during pregnancy, my colleagues and I studied the incidence of Covid-19 infection in pregnant women living in an area of Ecuador where transmission rates were high and screening was low. In our small study, published Wednesday in the journal Placenta, we found that a significant proportion of pregnant women we studied got infected with SARS-CoV-2. (I am an editor for Placenta but did not play an editorial role in publishing the paper.) Although these pregnant women responded well to the virus at the time of our study and did not develop severe illness, questions remain about the long-term impact of maternal coronavirus infection on the development of the fetus.

Intriguingly, we discovered that newborns whose mothers had Covid-19 had also developed an immune response to the virus. This suggests there’s active communication between the mother’s immune system and the fetus, and we argue this is further evidence in favor of vaccination.

Some of the protection developed by a vaccinated pregnant woman may be transmitted to her child during the pregnancy and after birth. This protection could be important for the newborn, since it provides the first line of defense in the child, who has an immature system. It’s also much safer for a mother to get vaccinated — which induces a short and specific immune response — than to potentially subject her body, and fetus, to fighting Covid-19 without protection. And the long-term impact of severe Covid-19 infection on a developing fetus is still unknown.

It’s understandable that pregnant women may feel uncertain about the Covid-19 vaccines. They should have been included in clinical trials of the vaccines last year, and their exclusion from medical research has been a longstanding issue.

There is no biological basis for concerns that the Covid-19 vaccines might have a negative impact on fertility or pregnancy, but this kind of hesitancy is what happens when pregnant people are left out of science.

What everyone should know is that there are a considerable number of pregnant women who have been vaccinated and protected against Covid-19 and have had healthy babies. Vaccination during pregnancy provides triple protection: to the mother, the developing fetus and the future health of the newborn.

Gil Mor is the scientific director of the C.S. Mott Center for Human Growth and Development. His research lab at Wayne State University studies the immune system during pregnancy and the impact of pathogens. He was president of the American Society for Reproductive Immunology from 2018 to 2021.

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