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The rise of a new ‘Deltacron’? BA.5 combines the worst traits of Omicron with the potential for severity reminiscent of Delta, experts say

Relatively new COVID-19 subvariant BA.5 takes some of Omicron’s worst traits—transmissibility and immune evasion—to a new level.

But it also combines them with a penchant for affecting the lungs reminiscent of the Delta variant that hit the U.S last summer and fall, according to two recent studies.

In the case of Delta, COVID tended to accumulate in and affect the lungs, potentially resulting in more severe disease. Until recently, a silver lining of Omicron has been its tendency to instead accumulate in the upper respiratory tract, causing symptoms more similar to a cold or the flu.

BA.5 is different, according to a study published June 10 on medRxiv, a Yale– and British Medical Journal–affiliated website that publishes studies not yet certified by peer review. Recent reports show BA.5 shifting back to the lower respiratory tract—at least in animal models—“with a potential increase in disease severity and infection within lung tissue,” researchers from Australia’s Kirby Institute wrote. They referenced another May preprint study that found BA.5 and close relative BA.4 replicate more efficiently in the alveoli of human lungs than so-called stealth Omicron, BA.2.

“BA.5 not only gives the virus greater antibody-evasion potential, but concurrently has changed [where it tends to accumulate], along with an increased transmission potential in the community,” the Kirby Institute authors write.

The scenario calls to mind the term “Deltacron,” which referred to a Delta-Omicron hybrid identified in the U.S. this spring that never took off. Back then, the term was used “prematurely,” Dr. Eric Topol, a professor of molecular medicine at Scripps Research and founder and director of the Scripps Research Translational Institute, wrote in a Sunday blog post on the studies.

Now, for BA.5, the term might be more appropriate, even though the subvariant isn’t a true hybrid.

“The ability to infect cells for BA.5 is more akin to Delta than the previous Omicron family of variants,” Topol wrote.

While BA.4 and BA.5 led to a wave of infections in South Africa this spring, they were relatively mild, with manageable rates of hospitalization. The U.S. and Europe, however, are currently seeing an increase in BA.5 hospitalizations, though well below levels seen in prior waves.

The difference could be due to the fact that South Africa saw a wave of the COVID variant Beta in late 2020/early 2021 that the West did not, potentially conferring better immunity to Omicron subvariants, Topol wrote.

BA.4 and BA.5—dominant in many countries across the globe—were first detected in the U.S. in late March, as Fortune previously reported, but have quickly taken over the global scene.

BA.5 alone caused about 54% of COVID infections in the U.S. two weeks ago, according to data released last week by the U.S. Centers for Disease Control and Prevention.

“The Omicron subvariant BA.5 is the worst version of the virus that we’ve seen,” Topol recently wrote. “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.

The jury is still out on whether current vaccines hold up against BA.5. But given that vaccines were 15% less effective against Omicron than they were against the Delta variant, even with a booster, “it would not be at all surprising to me to see further decline of protection against hospitalizations and deaths,” Topol wrote.

This story was originally featured on Fortune.com