XBB.1.5, ‘Kraken': Why is New COVID Variant So Contagious? What to Know as It Rises in Midwest

The new COVID variant XBB.1.5, also known unofficially as the "kraken" variant, continued to rise in the Midwest, according to data from the Centers for Disease Control and Prevention, but what makes the new variant so contagious and are experts expecting a surge?

The CDC has changed its estimates for how prevalent the agency believes XBB.1.5 is in the U.S. late last week, now saying the variant makes up just 27.6% cases, down from the roughly 40% of cases being reported. That would make it no longer dominant in the country, though it continues to make up a majority of cases in the Northeast, with the variant believed to be responsible for more than 70% of COVID infections.

In the Midwest, XBB.1.5 also rose in numbers to 7.3%, up from 6% of cases last week, CDC data showed.

Despite the change in estimates, the variant continues to be the only current omicron subvariant showing significant growth in recent weeks as experts say it is the most contagious variant yet.

So what makes it so transmissible?

The highly contagious "recombinant" variant is composed of two different BA.2 strains.

Dr. Ashish Jha, White House COVID czar, tweeted last week that the variant is likely more immune evasive, even "more than other omicron variants."

Maria Van Kerkhove, the WHO’s COVID-19 technical lead, described it as the most transmissible version of COVID yet.

Dr. Allison Arwady, commissioner of the Chicago Department of Public Health, said Tuesday that she anticipates numbers in the Midwest will continue to rise. She noted last week the variant is "clearly more infectious" than other variants, "meaning it's spreading more easily" and "outcompeting the other variants."

"I'm expecting to see some increase in cases," she said during a Facebook Live Tuesday. "New York is not getting overwhelmed with hospitalizations, which is great. They are up, not surprisingly, because cases are up, but again, we have tools, we have vaccines, we have treatments that can that can help keep this hopefully under control. So it wouldn't surprise me to see an increase in cases. It also wouldn't surprise me if we move from that medium to that high level. You know, I would love it if we didn't get there, but I am not anticipating, based on this variant, you know, the major surge where you're talking mask mandates or, you know, like when omicron first got here."

Experts believe a mutation allowing the variant to bind better to cells is behind its ability to spread so quickly.

"The virus needs to bind tightly to cells to be more efficient at getting in and that could help the virus be a little bit more efficient at infecting people," Andrew Pekosz, a virologist at Johns Hopkins University, told CNBC.

Jha echoed that finding, saying the variant "binds more tightly to the human ACE receptor," which can affect contagiousness.

While it's not clear where the variant originated exactly, Arwady said it has so far not shown signs of leading to more severe infections.

"They are seeing some increase in hospitalizations in older folks in the Northeast, but that seems to be at least based on what you know, I'm reading and hearing from folks, that seems to be based as much on the fact that they're seeing just a surge overall... and so with that increase, it seems like we're just seeing more older people get infected and if they're not up-to-date, especially with their vaccines, they are somewhat more likely to be hospitalized," she said during a Facebook Live last week. "But no, I haven't seen anything suggesting that this new subvariant is clearly making people sicker. I think we'll learn more as we follow it."

While there are significant concerns surrounding the new strain, it remains an omicron subvariant, Arwady added, which offers less risk than if the variant were in an entirely different family.

"At this point, it's just another subvariant," she said. "And the thing worries me the most, as always, is people who aren't up-to-date with their vaccine. We want people to be up-to-date and then other than that, it's just another version of COVID at this point."

Experts have said the bivalent COVID booster appears to offer some protection against the newer strains, particularly against severe illness, in large part because they remain under the omicron family.

"It does look like the vaccine, the bivalent booster is providing continued protection against hospitalization with these variants," Pekosz told CNBC. "It really emphasizes the need to get a booster particularly into vulnerable populations to provide continued protection from severe disease with these new variants."

But Jha noted that for those who had infections before July or who haven't received a bivalent COVID booster shot, protection is much lower.

"Right now, for folks without a very recent infection or a bivalent vaccine, you likely have very little protection against infection," he tweeted.