Earlier this year, all eyes were on Deltacrona frankenvirus, of sorts, that combined Delta and Omicron, and potentially the worst traits of both.
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 backif it ever left. This time its 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 variantwhich killed about 3.4%of those it infected, nearly double that of Omicron, according to a 2022 study published in Nature Reviews Immunology. It could also feature the record-setting transmissibility of Omicron.
But predicting the severity of such a strain is even more difficult because scientists arent sure exactly why Omicron causes less severe disease in many people, when compared to Delta.
XBCa combination of Delta and stealth Omicron BA.2 circulating in Asian countries like the Philippineshas 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. Its 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, its less likely to combine the easy transmissibility of Omicron with the deadly penchant of Delta to hide out in the lungs, experts say.
But they caution that scientists dont fully understand why COVID morphed from a lower respiratory disease in the days of, and prior to, Delta, to a less severe upper respiratory disease for many, as is the case with Omicron. The shift may involve changes outside the spike protein, which the virus uses to attach to and infect human cells. If so, its anyones best guess how future Deltacronseven with Omicron spike proteinswill present in terms of symptoms.
American Pi in the future?
Little is known about XBC, including what its symptoms are. Scientists are aware, however, that the new variant has spawned additional variants, Rajnarayanan points out.
Far less is known about XAY and XAW, first reported in South Africa and Russia, respectively. Other Deltacrons reported earlier this year, including XD in France and XS in the U.S., never took off, perhaps because they were less transmissible than stealth Omicron.
Regardless of how these Deltacrons play out this fall, theres always potential for another.
While Delta is rarely reported these days, its still being found in some places overseas , like China. And persistent infections in immunocompromised peoplewho may not be aware that theyre immunocompromised or that they have a long-term infectionmay mean Delta will live on, undetected, in patients worldwide.
In animals, Delta variantswith the potential to transmit back to the human populationhave been reported in more than half of U.S. states, usually in deer and mink. While the bulk of Delta infections in animals have been detected in North America, theyve also been reported in Lithuania, Switzerland, Hong Kong, Poland, Latvia, the Netherlands, Canada, South Africa, and more.
A combination of Delta and Omicron is not automatically worse, cautions Ryan Gregory, a professor of evolutionary biology at the University of Guelph in Ontario, Canada.
Deltas ability to dodge the immune systemwhich pales in comparison to that of the latest Omicron strainsin combination with Omicron severity isnt an altogether horrible prospect. Its not nearly as bad as that of a Deltacron with Deltas severity and Omicrons transmissibility and immune-escape capabilities, he said.
Scientists this fall are also keeping an eye on Delta strain AY.103, which picked up some mutations present in some Omicron strains as it evolved in mink and deer.
Such a varianttransmitted by the animals or immunocompromised patients it developed inhas the potential to be the next big thing in coronavirus evolution, the likes of which havent been seen since Omicron burst onto the scene roughly a year ago.
An American Pi might pop up like this, Rajnarayanan said, referring to the potential of AY.103 or a similar strain of Deltadifferent enough from Omicron to be granted its own Greek letter name by the World Health Organizationto rise to prominence.
Sign up for the Fortune Features email list so you dont miss our biggest features, exclusive interviews, and investigations.
