What we know about XBB, the new dominant COVID variant in New England

A resident received the new bivalent Omicron booster shot at the Hebrew Home at Riverdale, a nursing care facility in the Bronx, N.Y. The bivalent booster vaccine, which works against the Omicron variant as well as the original form of the virus, appears to be effective against XBB.ANDREW SENG/NYT

A new coronavirus variant dubbed XBB has swiftly become the dominant form of COVID-19 spreading in the Northeast, jumping from about 35 percent of cases during the week ending Dec. 17 to just over half of cases last week, according to CDC data.

Here’s a quick primer on what we know about the variant.

Is XBB more transmissible? Experts say the rapid spread of the XBB variant suggests it’s more adept than its predecessors at evading the immunity that comes from vaccines and infections.

“The most likely explanation is that it’s more transmissible,” said Dr. Jeremy Luban, professor of molecular medicine, biochemistry, and molecular biotechnology at UMass Chan Medical School, in a recent interview.

Who is at greatest risk? As with other recent variants, people who are immunocompromised face greater risk, and the monoclonal antibodies used to treat them do not work against the latest variants, including XBB. That has eliminated an important tool for treating some of the most vulnerable patients.

What can I do to protect myself? As always, experts urge people to get booster shots. The bivalent booster vaccine, which works against the Omicron variant as well as the original form of the virus, appears to be especially effective against XBB, according to a recent small study.

If I test positive for XBB, could I be looking at severe illness? While XBB does not seem to cause more severe illness or death, little is known about the effects of its subvariants, XBB.1 and XBB.1.5, which have turned up in the Northeast, said Rajendram Rajnarayanan, assistant dean of research at the New York Institute of Technology College of Osteopathic Medicine’s Jonesboro, Ark., campus, in a recent interview.

What are experts saying about XBB? In a Dec. 23 online column, Dr. Eric Topol, executive vice president of Scripps Research in La Jolla, Calif., noted that the arrival of XBB.1.5 in New York coincided with a marked rise in hospitalizations in that state.

“Of course, other factors are likely contributing such as waning of immunity, indoor/holiday gatherings, cold weather, lack of mitigation. But it is noteworthy that New York’s [COVID-19] hospital admission rate is the highest since late January,” he wrote. “So we don’t know for sure how much of this is being driven by XBB.1.5, but it doesn’t look favorable.”

Dr. Cyrus Shaphar, the White House’s COVID-19 data director, tweeted Dec. 23 that Americans can “maximize” their protection against XBB and the other variants by staying up to date with their vaccines. He also noted that the highest concentration of XBB can currently be found in the northeastern part of the country.

Variant update:
- ~100% of variants in US are Omicron subvariants
- BQ.1/BQ.1.1/XBB together comprise ~81% of circulating variants
-⬆️in XBB to ~18% nationally, highest proportions in the Northeast
- Maximize your protection this winter by getting an update bivalent vaccine! pic.twitter.com/OyySnaHscZ

— Dr. Cyrus Shahpar (@cyrusshahpar46) December 23, 2022

Topol, of Scripps, predicted on Tuesday that XBB will soon be the dominant variant nationwide.

“A functional @CDCgov would alert the public about the XBB.1.5 variant — which has already established dominance throughout the Northeast — and, with its big growth advantage over BQ.1.1, soon country-wide,” Topol tweeted.

A functional @CDCgov would alert the public about the XBB.1.5 variant—which has already established dominance throughout the Northeast—and, with its big growth advantage over BQ.1.1, soon country-wide. https://t.co/tL2Y2HF2ew

— Eric Topol (@EricTopol) December 27, 2022

The CDC says on its website that XBB is “a recombinant of two BA.2 sublineages.”

The University of Minnesota’s Center for Infectious Disease Research and Policy notes on its website that the CDC’s variant projections for the week ending Dec. 24 show “that the Omicron XBB variant, a recombinant of two BA.2 viruses, now makes up 18.3 percent of sequenced samples, up sharply from the week before. Much of the rise appears to be from two northeastern regions where XBB is now the dominant subtype. XBB has fueled outbreaks in parts of Asia, including Singapore.”

Regarding XBB, the center says, “experts are watching a subvariant called XBB.1.5 that was detected in New York and has a mutation that has been linked to immune escape. Scientists suspect that XBB.1.5 has a growth advantage over BQ.1.1.”

Felice Freyer and Zeina Mohammed contributed to this story. Material from prior Globe stories was used in this report.