Health officials are keeping a close eye on omicron subvariant XBB.1.5 due to its rapid growth in the U.S.
As of Dec. 31, the strain accounted for at least 40 percent of cases in the U.S. That's up from about 10 percent two weeks earlier, CDC estimates show. Meanwhile, hospitalizations have increased after briefly leveling off in December. The daily average for new hospital admissions was 6,499 as of Jan. 4, up from 5,510 a week earlier. The daily average for COVID-19 hospitalizations as of Jan. 4 was 46,534, up 12 percent over the last 14 days, according to HHS data compiled by The New York Times.
Here is what experts know and don't know about XBB.1.5 so far:
Where has it been detected?
The subvariant has been detected in at least 29 countries, officials from the World Health Organization said during a Jan. 4 press briefing. As more sequencing data becomes available, it may be detected in more countries, according to Maria Van Kerkhove, PhD, the WHO's COVID-19 technical lead.
In the U.S., it's most prevalent in the Northeast, where it accounts for more than 70 percent of cases across HHS regions that include New England and New York.
Is it more transmissible?
Dr. Van Kerkhove called it the "most transmissible subvariant that has been detected yet," due to mutations that allow it to adhere to the cell and replicate easily. Experts believe it has a growth advantage over its relative, XBB, which was already hailed as the most transmissible.
That doesn't mean that it's inherently more contagious, though, according to Ashish Jha, MD, the White House's COVID-19 coordinator. "It binds more tightly to the human ACE receptor. Might affect contagiousness," he wrote in a Jan. 4 Twitter thread on the XBB.1.5.
Does it cause more severe disease?
There's been no indication XBB.1.5. causes more severe disease than other omicron strains, though it's not completely certain yet.
"We do expect further waves of infection around the world but that doesn't have to translate into further waves of death because our countermeasures continue to work," Dr. Van Kerkhove said.
Will vaccines, treatments work against it?
It's not certain how well omicron boosters neutralize XBB.1.5, though more data is expected soon, Dr. Jha said.
"But right now, for folks without a very recent infection or a bivalent vaccine, you likely have very little protection against infection," from XBB.1.5, he said. "The new bivalent shot is your best protection against both infection and serious illness."
Read more on what recent studies have found about how well omicron boosters neutralize the BQ.1 strains and XBB here.
As far as treatments, Pfizer's antiviral Paxlovid regimen "should work fine based on what we know," Dr. Jha said.