The Centers for Disease Control and Prevention on Wednesday strongly recommended that pregnant and breastfeeding women be vaccinated against COVID-19, pointing to new safety data that officials hope will sway the many who have resisted despite mounting evidence that the coronavirus can pose grave risks to their health and increase the chance of a preterm birth.

The new guidance marked the first time the agency gave strong, unambiguous support to vaccination during pregnancy, bringing it in line with the advice of the American College of Obstetricians and Gynecologists and other medical specialty groups.

It comes amid a surge in COVID-19 infections across the nation, driven by the highly contagious delta variant, and renewed efforts by the Biden administration to push up vaccination rates to stem the virus’ spread. The Food and Drug Administration is expected to authorize a third vaccine dose for certain immunocompromised people as soon as Thursday.

“CDC encourages all pregnant people or people who are thinking about becoming pregnant and those breastfeeding to get vaccinated to protect themselves from COVID-19,” the CDC director, Dr. Rochelle Walensky, said. “The vaccines are safe and effective, and it has never been more urgent to increase vaccinations.”

Only 23% of pregnant women have received even one dose of COVID-19 vaccine in the United States, and, in recent weeks, physicians have reported seeing more pregnant patients becoming infected, CDC officials said.

The CDC said its new guidance applies not only to pregnant women but also to pregnant individuals who do not identify as female. (The surveillance data it reviewed, however, is based on participants who self-reported as female.)

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Pregnancy is on the CDC’s list of health conditions that increase the risk of severe disease for people infected with the coronavirus. They are significantly more likely than patients who are not pregnant to require intensive care, to be connected to a heart-lung bypass machine, and to require mechanical ventilation, and they face a 70% increased risk of dying.

Contracting COVID-19 can also increase the risk of the mother developing a dangerous condition called preeclampsia, and raise the risk of preterm births and stillbirths; severe COVID-19 disease further elevates these risks, and has been linked to gestational diabetes and low birth weight. In rare cases, the virus can be transmitted to the fetus during gestation.

Walensky pointed to new safety data that found no increased risk of miscarriage in a study of nearly 2,500 pregnant people who were immunized with one of the mRNA vaccines (the shots by Moderna or Pfizer and BioNTech) during the first 20 weeks of gestation. Earlier research found similarly reassuring data for those vaccinated later in pregnancy.

Some women remained conflicted about the decision.

“I’m highly considering getting vaccinated, I feel like it’s the more responsible thing to do,” said Raeshel Contreras, 29, who lives in the Bay Area. She is 28 weeks pregnant and worries that the vaccine could disrupt fetal development, though there is no scientific evidence of this. “I don’t know what I would do if I got vaccinated and something happened to the baby.”

She has given herself two weeks to make a decision, she said, but the timing of the new CDC directive, during a surge, concerned her.

“Why was this not recommended before?” she asked. “Now there’s this new variant, and now the CDC is jumping, but it wasn’t recommended, I’m like ‘Why is that?’”

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The data on birth outcomes is limited, considering the vaccine has only been available since December, but the small number of pregnancies of immunized obstetric patients followed to term have not identified any safety issues, according to Sascha R. Ellington, an epidemiologist who leads the emergency preparedness response team in the division of reproductive health at the CDC.

A study of 827 people who gave birth after being vaccinated found the rates of adverse events and poor outcomes were similar to prepandemic rates, but called for more long-term follow-up of such pregnancies. The study was published in April in the New England Journal of Medicine.

“At this time, the benefits of vaccination, and the known risks of COVID during pregnancy and the high rates of transmission right now, outweigh any theoretical risks of the vaccine,” Ellington said.

“I think it’s pretty clear with everything we’ve learned that you do have more severe disease in pregnancy, and there is concerning data from other countries that pregnant women are being more severely affected with the delta variant, compared with the earlier strains,” said Dr. Denise Jamieson, an obstetrician at Emory University in Atlanta.

Still, many pregnant patients, reluctant to put any foreign substance in their bodies, want more long-term data and scientific evidence that the vaccines will not have an effect on the development of the fetus, said Dr. Adam Urato, a maternal-fetal medicine specialist in Framingham, Massachusetts, who counsels patients about the vaccine.

“The one question my patients ask me all the time is, ‘Are we absolutely sure that these vaccines won’t affect my baby?’” he said.

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Tista Banerjee, 32, who gave birth to twins at the end of June, said she chose not to be vaccinated until after her pregnancy.

“During pregnancy they say that if you don’t have to take external medicines, don’t, and that you should be particular about what you put in your body,” Banerjee said. The vaccine was still quite new in April when she was considering vaccination, she said, and she was fortunate that she was able to work remotely and avoid unnecessary exposure to the virus.

She was fully vaccinated in July, soon after she gave birth, she said.

Pregnant women, often excluded from medical studies, were not included in the clinical trials of the COVID-19 vaccines, and the World Health Organization has been ambiguous in its guidance about vaccines, both for breastfeeding women, for whom it says there is no safety data, and for pregnant women.

In interim recommendations, issued in June, the global health organization said that it recommends vaccination “when the benefits of vaccination to the pregnant woman outweigh the potential risks.” The examples given were women who are at a high risk of being exposed to COVID-19, and those with chronic health conditions, like obesity or diabetes, that place them at higher risk for severe illness.