The issue of vaccinating pregnant women against COVID-19 is controversial because they were excluded from initial vaccine clinical trials.

In theory, vaccination would protect both mother and newborn because disease-fighting antibodies to the coronavirus would cross the placenta from the mother’s blood into fetal circulation.

But without solid data to support that theory, public health authorities and medical groups are divided about whether vaccination should be offered to pregnant women before studies prove it’s safe and effective for them.

A new study by University of Pennsylvania researchers, published Friday in JAMA Pediatrics, sheds some light. It finds that infected pregnant women transfer high levels of coronavirus antibodies to their newborns — a hint that vaccination might confer immunity on both.

Many questions remain to be answered. Here’s what is known and unknown so far.

How dangerous is COVID-19 for pregnant women and their babies?

Women are now routinely tested for COVID-19 when they go to the hospital to give birth. Many of them, research shows, are infected but have no symptoms. An analysis by the Centers for Disease Control and Prevention found slightly more than half of 548 hospitalized pregnant women who tested positive were asymptomatic.

The chance of severe COVID-19 in pregnant women is low — but significantly higher than in nonpregnant women, according to the CDC, perhaps because of “physiologic changes in pregnancy.”

COVID-19 is rare in newborns of mothers who had the disease while pregnant, suggesting perinatal transmission is uncommon. “Most newborns who tested positive for COVID-19 had mild or no symptoms and recovered,” the CDC says.

That is a reassuring contrast to some other viruses. Rubella (German measles) and the Zika virus readily pass to the fetus and can cause devastating defects.

Is there reason to think COVID-19 vaccination would be harmful for pregnant women?

The short answer is no. Even though pregnant women have been excluded from vaccine trials, a small number got pregnant during studies of the two authorized vaccinations, made by Moderna and Pfizer. Those women continued in the trials and no safety concerns emerged.

Both vaccines activate ultrahigh levels of antibodies to COVID-19 — far more than the body’s natural immune response.

“There is no data that suggests these vaccines cause harm in pregnant women — but there just isn’t a lot of data on this,” said Scott Hensley, a Penn microbiologist and co-author of the new antibody study.

It should be noted that some vaccinations, such as flu and whooping cough, are highly recommended for pregnant women. Pregnancy increases the chance of being hospitalized with influenza, and both flu and whooping cough can be deadly to infants.

What do we know about maternal and fetal immune responses to COVID-19?

Relatively little, which is why the Penn study is valuable.

Penn researchers tested blood samples from 1,471 women and umbilical cord blood from their newborns to look for antibodies that the body produces to fight COVID-19 infection. High levels of IgG — an antibody that lasts for months or years in other types of infection — were found in 83 women and 72 of their infants.

The 11 infants who didn’t show signs of antibody transfer may have had levels too low to detect, probably because their mothers were in the early days of infection, said study co-author Karen Puopolo, a neonatologist at Children’s Hospital of Philadelphia and associate professor of pediatrics at Penn’s Perelman School of Medicine.

“We were consistently able to detect IgG in cord blood if at least 17 days had passed since the infection began,” she said.

Among the remaining unknowns, Puopolo said, is what antibody level is protective for an infant and how long the protection lasts.

“We see our work as a first step,” she said. “Our work doesn’t comment on whether or not pregnant women should be vaccinated.”

Where do expert groups stand on that issue?

The World Health Organization recommends withholding COVID-19 vaccines from pregnant women unless they are at high risk of exposure to the virus, or have chronic health conditions that could worsen an infection.

But even the WHO says, “We don’t have any specific reason to believe there will be specific risks that would outweigh the benefits of vaccination for pregnant women.”

The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine stress that women who want to be vaccinated should not be denied “solely based on their pregnancy or lactation status.”

The CDC agrees, while recognizing that limited vaccine supplies may affect access: “If pregnant people are part of a group that is recommended to receive a COVID-19 vaccine (e.g., health-care personnel), they may choose to be vaccinated.”

The ob-gyns group, the maternal-fetal medicine society, the National Institutes of Health, and the National Academy of Medicine have consistently advocated for including pregnant and breastfeeding women in vaccine trials in general.

Pfizer plans to launch a clinical trial of its vaccine in pregnant women by June, while Moderna is establishing a registry to record outcomes in pregnant women, the New York Times reported this week.