‘Most transmissible’ COVID variant yet is spreading in California. What we know about XBB.1.5

Its rise had been most rapid in the Northeast, where XBB.1.5 made up 71.6% of sequenced coronavirus cases in the seven-day period ending Jan. 7, according to data from the U.S. Centers for Disease Control and Prevention. That compared to 27.6% across the U.S. as a whole, and 7.6% of cases in the Western region, which includes California.

The BQ.1.1 and BQ.1 variants continue to make up a majority of the coronavirus cases in the Golden State. Nationwide, BQ.1.1 also still maintains a slight edge, at 34.4%.

But last week, the World Health Organization called the XBB.1.5 omicron descendant the “most transmissible” variant yet, and at a briefing Tuesday urged travelers to wear masks on international flights to help slow its spread.

Here’s what is known so far about the XBB.1.5 variant, including its prospects for spreading further in California.

What is XBB.1.5?

XBB.1.5 is a sibling of the XBB variant, which is a recombinant from multiple BA.2 omicron subvariants — meaning that multiple variants combined to form a new one, “which can happen in hosts that are co-infected” with more than one version of the virus, explained Abraar Karan, an infectious disease doctor at Stanford.

The XBB variant caused small surges in Singapore and India last fall, said UCSF infectious disease expert Peter Chin-Hong.

“It was notorious for being one of the most immune resistant and evasive variants around, but wasn’t quite as good at holding on to and infecting cells,” he said, which is “probably the reason that it never really took off around the world.”

What is known about it so far?

What makes XBB.1.5 stand out is that it “retains the immune slipperiness of XBB, but also adds a mutation” that makes it easier to infect cells, Chin-Hong said. He likened it to a “bulldog that won’t let go.”

The mutation on the spike protein, known as F486P, allows it to better adhere to ACE2 receptors — the entry point into nose, throat and lung cells, according to Scientific American. That is “what is giving it its evolutionary advantage,” said UC Berkeley infectious disease expert John Swartzberg.

Some refer to it as the “Kraken” subvariant, coined by a professor in Ontario, equating its strength to the enormous mythical sea monster, Swartzberg said.

So far, XBB.1.5 does not appear to cause more severe illness, though research is not yet conclusive, experts say.

“It will likely not cause more severe illness than our more recent omicron variants given the waves of infection, as well as uptake of vaccinations and boosters in the U.S. as a whole,” Chin-Hong said. “However, in individuals who are older and unboosted, or immunocompromised with weakened or absent immune experience, it will potentially cause serious disease and deaths.”

Still, hospitalizations are increasing across the country, particularly in the Northeast, and more people are dying compared with a month ago, Swartzberg said.

“While this is associated with the emergence of XBB.1.5, we do not know if this subvariant is a sufficient cause,” he said. “If we had more robust surveillance, we would have a better understanding of its prevalence and trajectory.”

This will become clearer with time, Karan said, but for now, “this variant is outcompeting other variants and has mutations that give it an advantage to enter cells while also evading parts of our immune system.”

Will the strain likely become dominant here?

The CDC has reported a “doubling time” of nine days for XBB.1.5, referring to the number of days it takes for cases, hospitalizations and deaths related to a virus variant to double.

Experts all agree that the subvariant will likely become dominant across the U.S., perhaps by the end of the month, Swartzberg estimated.

“It is definitely here in the Bay Area already because of travel from the Northeast to the Bay Area that is common, but was amplified by holiday travel and get-togethers,” Chin-Hong said. “There is a also a lag before sequencing data is fully available, so it is likely already more common than the CDC data suggests in the Bay Area.”

Because of an “inadequate amount of PCR testing” and insufficient genomic sequencing of positive tests, Swartzberg said, “we have a very myopic vision as to how this virus is evolving.”

Still, the European CDC noted Tuesday that U.S. health officials had scaled back the estimated nationwide proportion of XBB.1.5 from 40% to the current 27.6%, leading to “significant uncertainty” about the subvariant’s prospects.

“It is still unclear whether the variant will become dominant in the U.S. in the coming few weeks,” the agency said. It furthermore noted that rapid growth in the U.S. would not necessarily mean the same would occur in Europe, “since major differences in variant circulation between North America and Europe have been observed several times during the pandemic.”

Will we see a major spike in COVID-19 cases from this subvariant?

Experts said it’s hard to predict, but while an increase in COVID cases is likely due to XBB.1.5, it is not likely to be a large surge.

“We won’t likely see a major spike in COVID cases, but a modest increase in cases in the community,” Chin-Hong said. “A smaller proportion of these will convert into hospitalizations compared with previous years in the Bay Area.”

He added that XBB’s surges in India and Singapore produced “manageable” hospitalizations.

Karan also doesn’t expect the same level of hospitalizations and deaths seen in 2022.

Swartzberg said the rise in cases will more likely be from people not taking COVID precautions, rather than the XBB.1.5 subvariant itself.

Do vaccines and treatments work against XBB.1.5?

CDC data so far suggests that the new bivalent boosters are more effective than previous vaccines in preventing symptomatic disease and hospitalizations, Karan said.

“While data hasn't been released on XBB.1.5 specifically, I would anticipate the same trends to hold,” he said.

Experts emphasize that while the vaccines may not be as effective at preventing infections, they still hold up well against serious illness and death.

The FDA recently said that it does not anticipate that the drug Evusheld will neutralize XBB.1.5. The drug is primarily used as a preventive therapy for immunocompromised people. Last month, physicians at UCSF were told to stop prescribing Evusheld and another monoclonal antibody treatment, bebtelovimab, because they are no longer effective against aggressive virus strains.

“This is a shame as this treatment was very important in protecting many immunocompromised individuals,” Swartzberg said.

That means immunocompromised people will have to rely on measures including wearing a high-quality mask in indoor public places, and being quickly prescribed COVID medications such as Paxlovid, molnupiravir or remdesivir if they do become infected.

What should people do right now?

Following the COVID health measures recommended throughout the pandemic continues to be the best course against XBB.1.5, experts said.

“This subvariant will be hard to avoid unless one is being careful,” Swartzberg said. “Being careful translates to doing the things that we know work,” which includes staying up to date on vaccines, wearing a good, well-fitted mask in indoor public spaces, and steering clear of others if you get sick.

Karan urges more people to get the updated bivalent booster, particularly those over 60 and people with severe comorbidities. Testing before big gatherings can also help prevent spread.

While most people who become infected will have a mild illness, Chin-Hong said, he is most worried about those who are over 65 and unboosted. Get a prescription for antiviral therapies such as Paxlovid if you become infected and you qualify, because it can help prevent hospitalization and lower the risk of long COVID, he said.

However, there’s “no need to panic,” Chin-Hong said. “Try to live life as normally as possible and take precautions when it makes sense based on who you are and the company you keep.”

He added: “Continue to have a flexible thinking and growth mindset.... We have made a lot of progress and have a lot of tools that can keep everyone safe. It is up to us to use them.”

Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang