Around 70 percent of health-care workers worldwide are women, meaning many are on the front lines of the coronavirus pandemic and, in theory, toward the front of the line for a vaccine shot as they begin to be distributed.

But there’s a catch: Vaccination programs from Russia to the United Kingdom are excluding women who are pregnant, breastfeeding or planning for a pregnancy because the vaccine candidates were not clinically tested on these populations as part of their initial trials.

On Tuesday, Britain launched the West’s first mass coronavirus vaccinations using the Pfizer-BioNTech shot. But guidance so far from the U.K. government has excluded pregnant women from the shots.

“Evidence so far reviewed … raises no concerns for safety in pregnancy,” Public Health England explained on its website. But “because of the new formulation of this particular vaccine,” the agency “wants to see more non-clinical data before finalising the advice in pregnancy.”

The advisory continued: “It is standard practice when waiting for such data on any medicine, to avoid its use in those who may become pregnant or who are breastfeeding. This will be kept under review as more evidence becomes available.”

Russia’s recent initial rollout of its Sputnik V vaccine similarly did not include this demographic.

Medical professionals acknowledge that — given the lack of available scientific data on how coronavirus vaccines interact with pregnancies — there is not yet sufficient evidence to safely recommend their use. But some also describe that as a symptom of a larger problem and are urging trials to begin including pregnant and breastfeeding women to ensure equitable coverage going forward.

“The lack of vaccination data on pregnant and breastfeeding people is definitely important as it means it’s not truly inclusive data, but also — women make up a significant portion of the healthcare workforce,” Saskia Popescu, an infectious-disease epidemiologist and infection preventionist at the University of Arizona and an external expert advising the European Center for Disease Prevention and Control, wrote in an email.

She continued: “Moreover, we have seen that pregnant women are at risk for severe disease and preterm birth as a result of SARS-CoV-2 infection. The vaccines are still very new but we really need to be mindful of these knowledge gaps and that moving forward, we will need to address them both from a safety standpoint, but also an ethical one.”

The vaccine gap could additionally have employment implications at a time when pandemic-related job losses have disproportionately hit women hardest. What if, for example, a doctor or nurse has to choose between a possible pregnancy and a vaccine to continue their work?

Pregnant women and children have historically been excluded from initial vaccine trials because they are classified as more vulnerable populations, said Alyson Kelvin, a flu and infectious-disease researcher at the Canadian Center for Vaccinology and Dalhousie University.

Women while pregnant are “immunologically different” to protect the developing fetus, said Kelvin, so “pregnant women might have a different response” to a vaccine “when she’s pregnant versus when she’s not pregnant.”

At the same time, women 28 weeks or more into a pregnancy are considered at increased risk if they contract viral illnesses such as the coronavirus, raising the stakes of their not being included in trials.

Pregnant women were similarly left out of initial studies of influenza vaccines. Over time, as flu vaccines were shown to be safe in a wide range of populations, the guidance changed to clear pregnant and breastfeeding women for the vaccines. In these cases, researchers also comb through data to analyze how vaccines may have interacted with women in the studies who did not know or intend to be pregnant — which could have been the case for some participants in the coronavirus vaccine studies as well.

But Marian Knight, a professor of maternal and child population health at the University of Oxford, said the coronavirus pandemic offers a clear case for changing this regulatory framework.

“The precautionary principle when excluding pregnant and breastfeeding women from research doesn’t ever think about how they might benefit from the research,” she said. “The default should be inclusion, unless there’s a clear reason that women should be excluded.”

The U.K.’s Royal College of Obstetricians and Gynaecologists has called on the government “to fund research studies to establish the suitability of any approved coronavirus vaccines in pregnant and breastfeeding women.”

Ruth Faden, founder of Johns Hopkins University’s bioethics institute, similarly urged the U.S. Food and Drug Administration in a recent letter to permit pregnant and lactating women to receive the Pfizer vaccine while acknowledging the unknown safety implications.

Still, Knight said she expected it would be some time before coronavirus vaccines are licensed for pregnant women, as there would need to be several phases of trials conducted to gather the necessary data.

“Pregnant women, who we know are also at risk, have no means of protection,” she said. “So the only thing we can be advising pregnant women is to continue social distancing and being very diligent about it. Whereas for all others [at high risk], the vaccine is really the beginning of seeing some of our freedoms back.”

Meryl Kornfield contributed to this report.