Frederick Morley, a Hartford Healthcare nurse, grabs a COVID-19 vaccine syringe to inject a patient. Yehyun Kim /

The arrival of COVID-19 mRNA vaccines in late 2020, while widely celebrated, brought a new wave of vaccine misinformation.

Dr. Alice Lu-Culligan, of the Department of Immunobiology at Yale, said this was especially true among young adults of child-bearing age and pregnant women, the latter of whom are regularly excluded from clinical trials and safety data.

“One theme that we kept coming across was the rampant misinformation surrounding infertility and the vaccines and also the dangers of vaccinating during pregnancy,” Lu-Culligan said.

After studying human blood samples and conducting experiments in pregnant mice, researchers at the Yale School of Medicine found no evidence that the vaccines are associated with difficulties in becoming pregnant or cause any birth defects and growth problems.

The results add to a growing body of research from scientists around the world who have come to similar conclusions. Experts hope to use the findings to boost trust in the vaccines’ safety and dispel some popular vaccine fears.

“To have been able to rigorously test it scientifically and address the rumor with science is also really valuable,” said Alexandra “Sasha” Tabachnikova, a second-year Ph.D. student at Yale’s Department of Immunobiology. “Having the science to say, ‘No, we didn’t see evidence of that’ is valuable.”

Lu-Culligan and Tabachnikova were members of the Yale research team led by Akiko Iwasaki, an immunobiologist and national expert during the pandemic. Results of the team’s study were published May 24 in PLOS Biology.

Researchers injected high doses of COVID-19 mRNA vaccines into mice at the earliest stages of pregnancy, to look at possible effects of the vaccines on fetal development.

Experiments like this in early-stage human pregnancies are rare, Lu-Culligan clarified, as there are ethical barriers. Many women also remain unaware of their pregnancies until later points of gestation.

Researchers studied the mice through pregnancy and birth and found that the offspring were born healthy at normal weight and size, with no abnormalities.

Different studies that follow human pregnancy and fetal development among vaccinated people are ongoing. Results from completed studies so far show no evidence that the vaccines cause poor pregnancy and birth outcomes.

The Yale team also sought to address unproven theories that mRNA COVID-19 vaccines created heightened levels of antibodies that would attack a human protein called syncytin-1, a protein crucial to the development of the placenta.

Researchers studied blood samples from 96 vaccinated and unvaccinated people. Tabachnikova said the team did not find higher levels of antibodies in women who were vaccinated compared to those who did not receive a vaccine.

The scientists were therefore able to debunk unfounded claims that mRNA vaccines could result in infertility.

“This is also supported by what we know in the real world,” Lu-Culligan said, “which is that there have been no reports to date in humans that there has been infertility associated with mRNA vaccination.”

Lu-Culligan said while the study was robust, there’s always more research to do when it comes to understanding human response to infections and vaccinations.

And, she added, given the pervasive culture of vaccine misinformation, the need for scientific research is far greater now.

“A lot of times, the scientific and medical communities, we often hear this misinformation and kind of feel helpless,” Lu-Culligan said. “This is a demonstration that we can answer some of these questions, and they deserve to be answered if a lot of people are asking them and are concerned.”

The U.S. Centers for Disease Control and Prevention recommends COVID-19 vaccines for nearly everyone 5 years and older, including pregnant people.