The initial strain, reported in January, failed to take flight. Reports of additional Delta-Omicron hybrids, in multiple locations across the globe, later emerged, then fizzled.

But in the waning days of 2022, the Deltacron phenomenon is back—if it ever left. This time it’s in the form of new COVID variants XBC, XAY, and XAW.

In a worst-case scenario, a Delta-Omicron hybrid could be as deadly as the Delta variant—which killed about 3.4% of those it infected, nearly double the fatality rate of Omicron, according to a 2022 study published in Nature Reviews Immunology. It could also feature the record-setting transmissibility of Omicron. 

Predicting the severity of such a strain is difficult because scientists aren’t sure exactly why Omicron seems to cause less severe disease in many people, when compared to Delta.

XBC—a combination of Delta and “stealth Omicron” BA.2 circulating in Asian countries like the Philippines—has the greatest potential in the group for transmission, variant tracker Raj Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., tells Fortune. It’s been identified in the U.S. but only in seven cases so far, mainly from California.

Because XBC contains the viral body of Delta and the spike protein of Omicron, it’s less likely to combine the easy transmissibility of Omicron with the deadly penchant of Delta to hide out in the lungs, experts say.