Meet ‘Centaurus,’ the new ‘stealth Omicron.’ It was just found in the U.S. and may escape immunity more than any other COVID strain

A new Omicron subvariant on the radar of the World Health Organization—one some experts say could be the most immune-evasive yet—has been identified in the U.S., the Centers for Disease Control and Prevention told Fortune on Thursday.

There have been two cases of BA.2.75, dubbed “Centaurus,” detected in the U.S., with the first being identified on June 14, a spokesperson for the CDC said.

The CDC does not publicly report on emerging variants until they comprise 1% of cases. Thus, current cases of BA.2.75 are being reported on the agency’s data tracker under BA.2 cases, which comprised less than 3% of reported U.S. cases last week, according to data released on Tuesday.

Centaurus has recently risen to prominence in India, competing with the BA.5 Omicron subvariant that is sweeping the globe. WHO officials said they were tracking the ultra-new subvariant at a Wednesday press conference and released some information about it via Twitter on Tuesday.

BA.2.75 has been reported in “about 10 other countries” and has not been declared a variant of concern, Dr. Soumya Swaminathan, WHO's chief scientist, said in a Tuesday tweet. Transmissibility, severity, and potential for immune evasion are currently unknown, she added.

But some experts are raising potential red flags. Dr. Eric Topol, a professor of molecular medicine at Scripps Research and founder and director of the Scripps Research Translational Institute, said Monday the new subvariant’s mutations “could make immune escape worse than what we’re seeing now” with BA.5 and BA.4, both of which are subvariants known to evade immunity from both vaccination and prior infection.

BA.2.75 was first detected in India in early June. Along with the usual Omicron mutations, it has as many as nine additional changes, none of which are concerning individually. “But all appearing together at once is another matter,” Tom Peacock, a virologist at the Department of Infectious Disease at Imperial College in London, said recently in a tweet.

Its “apparent rapid growth and wide geographical spread” are concerning, he added.

Aside from India, the virus has been detected in Australia, Canada, Germany, New Zealand, and the U.K., according to a Tuesday statement by the University of Minnesota’s Center for Infectious Disease Research and Policy, citing Ulrich Elling, a researcher with Austria’s Institute of Molecular Biotechnology.

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told Fortune on Thursday that it’s unclear if Centaurus can “really take off” in the face of BA.5 and relative BA.4.

Centaurus “may just spread for some period of time until it runs into BA.5 and is outcompeted for people to infect,” Adalja said. “I don’t know at this time that BA.2.75 will be anything more than a regional issue that eventually gets overwhelmed by BA.5.”

The ultra-new variant could also mirror another “stealth Omicron” spinoff, BA.2.12.1, in that it could take over for a period—as BA.2.12.1 did in the U.S., becoming dominant over BA.2 in May and remaining dominant until BA.4 and BA.5 pushed it down in late June—until the next more transmissible variant comes along, he said.

As to whether Centaurus might cause more severe disease, such variants are “not going to be something evolution pushes for,” he said, adding that those with more severe disease are typically at home or in a hospital, too sick to go out and spread the virus.

BA.5 is now dominant in the U.S. The previous heavy hitter, BA.2, is now a shadow of its former self.

“The Omicron subvariant BA.5 is the worst version of the virus that we’ve seen,” Topol wrote last week as the subvariant was well on its way to becoming dominant in the U.S. “It takes immune escape, already extensive, to the next level, and, as a function of that, enhanced transmissibility,” well beyond what has been seen before.

A recent study out of South Africa found that those who had been previously infected with Omicron but not vaccinated experienced a nearly eightfold drop in neutralizing antibodies when exposed to BA.4 and BA.5. Those who had been vaccinated and previously infected with Omicron saw a milder threefold decrease.

This story was originally featured on Fortune.com