A new study that found no evidence of injuries in the placentas of patients who received a COVID-19 vaccine while pregnant is another sign that the shots are safe for this population, researchers said. The study, conducted by Northwestern Medicine and published in Obstetrics & Gynecology, is believed to be the first to analyze potential vaccine impact on placentas.

"The placenta is like the black box in an airplane," Dr. Jeffery Goldstein, assistant professor of pathology at Northwestern University Feinberg School of Medicine, and Northwestern Medicine pathologist, said in a news release. "If something goes wrong with a pregnancy, we usually see changes in the placenta that can help us figure out what happened. From what we can tell, the COVID vaccine does not damage the placenta."

For the research, the team compared 84 placentas from vaccinated patients with 116 placentas from unvaccinated patients. All patients delivered babies at Prentice Women’s Hospital in Chicago, according to the news release, which was published on EurekAlert.org. Most of the vaccinated patients received either the Moderna or Pfizer shots during their third trimester.

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The team looked for abnormal blood flow between the mother and fetus, and issues with fetal blood flow, as both issues have been reported in pregnant patients who test positive for coronavirus. The rate of such injuries was the same in vaccinated patients and unvaccinated patients.

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"We have reached a stage in vaccine distribution where we are seeing vaccine hesitancy, and this hesitancy is pronounced for pregnant people," Dr. Emily Miller, Northwestern Medicine maternal fetal medicine physician and assistant professor of obstetrics and gynecology at Feinberg, and study co-author, said. "Our team hopes these data, albeit preliminary, can reduce concerns about the risk for the vaccine to the pregnancy."

When the currently-authorized vaccines were submitted to the FDA and CDC for approval, there was not enough data on pregnant patients for regulators to issue formal guidance regarding this population. Since then, multiple studies have shown the vaccine to be safe and effective in pregnant women, including data that revealed viral antibodies can be passed on to the fetus via vaccination of the mother.

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"The Internet has amplified a concern that the vaccine might trigger an immunological response that causes the mother to reject the fetus," Goldstein said in the news release. "But these findings lead us to believe that doesn’t happen."