Researchers in Japan say a new coronavirus variant thought to be better at evading the human body's immune system is becoming increasingly prevalent nationwide.

The National Institute of Infectious Diseases says the JN.1 variant accounts for an estimated 31 percent of all detected cases this week, up sharply from 11.6 percent in the week through December 3.

JN.1 is a sub-variant of the Omicron strain. It has recently been spreading rapidly across the globe, and the World Health Organization designated it as a "variant of interest" last week.

The WHO says it may have an enhanced ability to evade the immune system, but poses a low risk of serious infection.

In the United States, the Centers for Disease Control and Prevention says JN.1 accounted for an estimated 44 percent of new cases reported in the two weeks through December 23. That's up about 20 percentage points from the preceding two weeks.

The WHO says the variant has been reported in 41 countries as of December 16, including France, Singapore, Britain and Sweden.

Professor Hamada Atsuo of Tokyo Medical University says the coronavirus is now spreading faster in some countries where JN.1 is becoming more prevalent.

Hamada is urging people in Japan to take thorough anti-infection measures.