Children under the age of 18 who are unvaccinated against the coronavirus, and a limited category of foreigners arriving from countries with low vaccination rates, are among the travelers exempted from forthcoming requirements that will determine who can enter the United States, Biden administration officials said on Monday.

The Biden administration has announced that it would lift travel restrictions on Nov. 8 and reopen the United States to fully vaccinated international travelers who had been barred for nearly a year and a half from entering the country by air or crossing the land borders.

But the new travel system also comes with stringent requirements, and will seal the United States off from most foreigners who have not yet received a vaccine cleared by the World Health Organization or the Centers for Disease Control and Prevention. On Monday, senior officials detailed opportunities to enter the United States for certain travelers who struggled to obtain a vaccine because of a lack of uniform vaccine eligibility for minors, as well as limited access to the global supply.

Unvaccinated children under the age of 18 will be permitted to enter the United States when the new system takes effect, officials said. Children older than 2 who are traveling with an adult will need to show a negative coronavirus test within three days of their departure date. Those traveling alone will need to show such a result one day before they travel to the United States.

The exemptions will also apply to adults flying from countries where less than 10 percent of the overall population is fully vaccinated, if they can show a “compelling reason” for entering the United States, officials said. That carve-out, they said, would apply to a narrow group of unvaccinated travelers; entering the United States for tourism would not clear the bar for an exemption.

Others who show a U.S. government-issued letter approving an emergency or humanitarian need to travel will also be allowed to cross U.S. borders.