XBB—quite possibly the most immune-evasive COVID variant to date—has been identified in 26 countries, the World Health Organization reported Wednesday.

The organization did not name the countries in which it had found XBB. Levels of the variant have been rising in Singapore this month, despite a broadly vaccinated and boosted population, as well as in Bangladesh. But cases are also rising in other countries, including Canada, England, and India, according to data from GISAID, an international research organization that tracks changes in COVID and the flu virus.

GISAID data shows the largest concentration of XBB or spinoff variants over the past 30 days in Singapore, followed by India, Bangladesh, the U.S., Australia, and Denmark.

XBB is a combination of two different strains of Omicron. It’s in competition with fellow Omicron spawn BQ.1.1 for the title of most immune-evasive variant so far, surpassing that of shared ancestor BA.5, which was dominant across the globe this summer. But the WHO report suggested that XBB is indeed the most antibody-evasive variant yet, citing “laboratory-based evidence.”

While XBB also seems to spread more effectively than other Omicron variants, it’s yet known if it causes more severe disease than other strains, the report added.

An evolving threat

XBB has not been identified in the U.S. so far. But spinoffs of it—XBB.1, XBB.2, and XBB.3—have, according to GISAID data.

XBB.1 was first detected in the U.S. on Sept. 15, and represented 0.26% of cases genetically sequenced over the past 15 days, Raj Rajnarayanan, an assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., told Fortune on Monday, citing GISAID data. Most XBB strains had been found in New York State.

Worldwide, nearly three times as many cases of XBB.1 had been identified as cases of XBB, the WHO said this week.

It’s currently unclear what advantages, if any, XBB descendants like XBB.1 might have over their parent variant. Compared with XBB, XBB.1 features just “one small change” to the spike protein, which the virus uses to attach to and infect cells. The impact of the alteration is unknown, Rajnarayanan said.

XBB, in addition to competitor BQ.1.1, escape antibody immunity, rendering useless monoclonal antibody treatments used in high-risk individuals with COVID. According to a study last month out of Peking University’s Biomedical Pioneering Innovation Center in China, both escape immunity from bebtelovimab, the last monoclonal antibody drug that is effective on all variants, as well as Evusheld, which works on some. And both could lead to more severe symptoms, the authors wrote.

The ability of XBB to evade immunity is “extreme,” approaching the level of immune evasion shown by SARS, a coronavirus that infected thousands and caused nearly 800 deaths in the early 2000s, the authors added.

Scientists, including top U.S. infectious disease expert Dr. Anthony Fauci, expect a fall and winter wave of cases in the U.S. that begins to surge this month and peaks in January. It’s still unclear which COVID variant may fuel that wave, if any one variant does. But XBB and BQ.1 are top contenders, experts say.