India, June 17 -- Just before the busy summer travel season in the US, a new COVID variant called NB.1.8.1-also known as "Nimbus"-is quickly spreading across the world. This version of the virus, which is part of the Omicron family, recently caused a rise in cases in parts of Asia. Now, it's responsible for over one-third of all COVID infections in the US, according to new data.

Health officials first found the variant in the US in late March through testing international travelers at airports. Since then, it's been spotted in over a dozen states, and numbers are expected to grow.

On May 23, the World Health Organization called NB.1.8.1 a "variant under monitoring" because of how fast it's spreading and changes in its spike protein that...