The CDC's Nowcast data tracker, which displays COVID-19 estimates and projections for two-week periods, reflected that the KP.3.1.1 variant accounted for 57.2% of positive infections, followed by XEC at 10.7% in the two weeks starting on Sept. 29 and ending on Oct. 12.

KP.3 was the 'predominant' SARS-CoV-2 variant in the US. It was also spreading in Europe. It's now joined with another variant and become XEC.

The CDC is "monitoring the XEC variant," Rosa Norman, a CDC spokesperson told USA TODAY. This variant "is the proposed name of a recombinant, or hybrid, of the closely related Omicron lineages KS.1.1 and KP.3.3."