WASHINGTON — The head of the Centers for Disease Control and Prevention says new COVID-19 cases and hospitalizations are down in recent weeks, but three mutations that are causing concern have been detected in the U.S.

Dr. Rochelle Walensky said Monday at the White House coronavirus briefing that most of the dozens of U.S. cases of coronavirus mutations, or variants, involve the strain first detected in the United Kingdom.

But three cases involving a worrisome mutation first detected in South Africa have also been confirmed, as well as one case involving a strain first detected in Brazil.

The UK strain spreads more easily and is believed to be deadlier, but the South Africa strain is prompting even more concern because of early indications that vaccines may not be as protective against it.

Walensky urged Americans to get vaccinated as soon as shots become available to them, and stressed it’s no time to relax basic precautions such as wearing masks.