Researchers at the Dallas hospital found that in late August and early September, 10% to 15% of pregnant patients who tested positive for COVID-19 needed to be hospitalized for severe illness.
That was more than double the percentage of a year ago.
"We had an influx of severe illness that came rapidly," said lead researcher Dr. Emily Adhikari, an assistant professor of obstetrics and gynecology at UT Southwestern Medical Center.
And all but one of those hospitalized patients, her team found, were unvaccinated.
The findings, published online recently in the American Journal of Obstetrics and Gynecology, come on the heels of an alert from the U.S. Centers for Disease Control and Prevention urging pregnant women to get vaccinated against COVID-19.
The advisory, issued Sept. 29, said that during the pandemic more than 22,000 pregnant patients had been hospitalized for COVID-19 in the United States, and 161 had died. Of those deaths, 22 occurred in August alone â€” the highest number in a single month during the pandemic.
At this point, just under one-third of pregnant U.S. women are vaccinated. The CDC called for "urgent action" to get that number up.
Adhikari said her team's findings align with the CDC's concerns.
"This is real," she stressed.
Dr. Aaron Glatt, an infectious disease specialist not involved in the study, said the findings offer important information.
"Clearly, it's very smart to get vaccinated," said Glatt, a spokesman for the Infectious Diseases Society of America and chief of infectious diseases at Mount Sinai Hospital Nassau, in Oceanside, N.Y.
"If you're pregnant and you get COVID-19," he said, "you're at increased risk of becoming severely ill."
That was true before the emergence of Delta: According to the CDC, pregnant COVID-19 patients are known to be at heightened risk of needing hospitalization, intensive care and ventilators or other special equipment to help them breathe.
Despite that, many pregnant women remain hesitant to accept the vaccine.
In the beginning of the vaccination campaign, there was little hard data to give them. But that has changed, and studies that have followed vaccinated and unvaccinated pregnant patients have been "reassuring," said Kirstie Perrotta, an information specialist with MotherToBaby.
MotherToBaby is a free service run by the Organization of Teratology Information Specialists, a professional society of experts on environmental exposures and birth defects. It offers science-based information on the safety of medications, vaccinations and other exposures during pregnancy.
"Right now, COVID vaccination is the number one topic we're getting questions on," Perrotta said.
Based on the studies so far, she said, there is no evidence COVID-19 vaccination boosts the risks of pregnancy complications like miscarriage, stillbirth or preterm delivery.
Nor is there evidence of effects on fertility, Perrotta said â€” which is among the questions people have.
Another common worry is whether vaccination has potential long-term effects on the baby. There are no data on that, but Perrotta said experts believe it's unlikely there would be any such risks.
For one, the vaccines do not cross the placenta, she noted. And based on what's known about other vaccines given during pregnancy, like the flu shot, there is no reason to believe COVID-19 vaccination would harm babies.
Adhikari said that, in general, there's no reason to expect long-term risks from vaccines, as they do not linger in the body.
The study involved 1,515 pregnant patients diagnosed with COVID-19 at Parkland Health & Hospital System, the public hospital for Dallas County. Between May 2020 and Sept. 4, 2021, 82 of those patients required hospitalization.
The proportion of patients who became that ill increased after the rise of the Delta variant. Last year, the percentage of pregnant patients who required hospitalization hovered around 5%. By August and early September of this year, that had risen to between 10% and 15%.
Only one hospitalized patient had been vaccinated against COVID-19.
COVID-19 vaccines do not provide perfect protection against infection, but they greatly reduce the odds of becoming very ill, Adhikari said.
For more on COVID-19 vaccination and pregnancy, visit MotherToBaby.American Journal of Obstetrics and Gynecology