Because they might not be vaccinated with shots recognized by the U.S. Food and Drug Administration or the World Health Organization.

When lifting overseas travel restrictions in November, the U.S. required adults coming to the country to be fully vaccinated with shots approved or authorized by the FDA or allowed by WHO.

Among the most widely used vaccines that don't meet that criteria are Russia's Sputnik V vaccine and China's CanSino vaccine. Sputnik V is authorized for use in more than 70 countries while CanSino is allowed in at least nine countries. WHO still is awaiting more data about both vaccines before making a decision.

Vaccines recognized by the FDA and WHO undergo rigorous testing and review to determine they're safe and effective. And among the vaccines used internationally, experts say some likely won’t be recognized by the agencies.

“They will not all be evaluated in clinical trials with the necessary rigor,” said Dr. William Moss, executive director of the Johns Hopkins International Vaccine Access Center.

An exception to the U.S. rule is people who received a full series of the Novavax vaccine in a late-stage study. The U.S. is accepting the participants who received the vaccine, not a placebo, because it was a rigorous study with oversight from an independent monitoring board.

The U.S. also allows entry to people who got two doses of any “mix-and-match” combination of vaccines on the FDA and WHO lists.