The Biden administration outlined on Monday very narrow exemptions that will permit unvaccinated international travelers to enter the United States.

Anyone who is under the age of 18 traveling from overseas will need to show a negative COVID-19 test before boarding a flight, but they are exempted from vaccination requirements, the White House said.

Even though there are vaccines available in the U.S. to children as young as 12, administration officials said they are sensitive to the global variability regarding access to vaccination for older children who are otherwise eligible to be vaccinated.

Similarly, the White House said people who are traveling on non-tourist visas from countries that have vaccinated less than 10 percent of their population are also exempt from the vaccine requirement. There are more than 50 countries that meet that threshold, including much of Africa, according to the World Health Organization.

“They need to have a specific, compelling reason. So, tourist visas will not qualify for that,” a senior administration official said. “If they qualify, they need to show proof of a negative test taken within 24 hours prior to departure.

Other exemptions include those with certain medical conditions, clinical trial participants and those traveling on short notice for emergency or humanitarian reasons, the official said.