Citizens of six predominantly Muslim countries will be barred from acquiring visas to the United States, as part of a new executive order on immigration signed by President Trump today. Unlike a previous executive order, which is on hold, this 90-day freeze will not apply to current visa holders or citizens of Iraq. However, it raises concerns for thousands of foreign-born doctors currently working in the United States, as well as others hoping to work here in the near future.

There are more than 7,000 physicians working in the U.S. who trained in Iran, Libya, Somalia, Sudan, Syria and Yemen, the six countries in Trump’s new executive order, according to research from a group of doctors and economists at Harvard Medical School using data from Doximity, a professional network for physicians. The Association of American Medical Colleges, which represents all of the accredited medical schools in the U.S. and most teaching hospitals, calculated how many people were affected by the previous executive order covering seven countries, estimating that figure at more than 10,000. The organization says there are also 80 students from those seven countries currently enrolled in U.S. medical schools and about 1,000 people who have applied for residencies and fellowships.

In light of the revised ban, graduate students from Harvard and MIT used Doximity data to try to quantify the role those physicians play in the U.S. medical system and who they serve. They found that doctors from the countries subject to the ban are concentrated in Rust Belt states and Appalachia, and estimated that they account for more than 14 million patient visits a year. The doctors are more likely to work in underserved areas and more likely to practice in areas of medicine facing shortages, such as pediatrics and psychiatry, meaning that many likely play a vital role in vulnerable communities.

It’s unclear what effect the ban will have on these doctors, but Trump’s backlash against immigration already appears to be affecting other scientific fields. Graduate engineering programs have seen as much as a 30 percent decline in applications this year. And the earlier iteration of the ban resulted in physicians, residents and their families being denied entry to the U.S.

Michael Stepner, a graduate student at MIT, worked on the research exploring these doctors’ role in U.S. medicine. He said that it won’t be easy to find replacements for doctors restricted by the travel ban who otherwise would have worked in underserved areas. “These are areas that have been losing jobs and population. A lot of the highly skilled people [like physicians] leave for new opportunities, and the remaining people are left with few options,” he said.

Stepner acknowledged that his team was taking an activist stance on the issue; their website provides resources for people who want to call their Congress member to oppose the ban. He said the months- to yearslong timeline for publishing most academic research didn’t feel appropriate given the immediacy of the executive order.

“We don’t think about this as part of our academic research,” he said.

CORRECTION (March 6, 7 p.m.): An earlier version of this article misstated the name of an organization that represents medical schools. It is the Association of American Medical Colleges, not the American Association of Medical Colleges.