The CDC said it is too soon to know whether this might cause more severe illness compared with previous variants. But due to the high number of mutations detected in this lineage, there were concerns about its impact on immunity from vaccines and previous infections, the agency said.

"The large number of mutations in this variant raises concerns of greater escape from existing immunity from vaccines and previous infections compared with other recent variants," the CDC stated in its assessment.

"For example, one analysis of mutations suggests the difference may be as large as or greater than that between BA.2 and XBB.1.5, which circulated nearly a year apart."

But it said that "virus samples are not yet broadly available for more reliable laboratory testing of antibodies, and it is too soon to know the real-world impacts on immunity."

The agency added that it detected at least two cases with the BA.2.86 variant in the United States, although few other details were provided. It was also found in Israel, the United Kingdom, South Africa, and Denmark, the agency said.

One of the BA.2.86 cases was found in a person detected via the CDC's traveler surveillance system, while it added that cases being found in several countries is evidence of international transmission.

"Notably, the amount of genomic sequencing of SARS-CoV-2 globally has declined substantially from previous years, meaning more variants may emerge and spread undetected for longer periods of time," according to the assessment.

"It is also important to note that the current increase in hospitalizations in the United States is not likely driven by the BA.2.86 variant. This assessment may change as additional data become available."