The newest strain of the novel coronavirus may be the most transmissible yet.

The latest variant, named XBB.1.5, is an offshoot of the BA.1 variant of Omicron. Since the first case was reported in New York City around Thanksgiving, XBB.1.5 has spread rapidly, increasing from 2% of U.S. COVID-19 cases in December to 27% in early January.