A new coronavirus variant named 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 last week, according to CDC data.
The rapid spread indicates that the XBB variant is more adept than its predecessors at evading the immunity that comes from vaccines and infections.
“It looks like it’s just going to blow the other ones away in a very short period,” said Dr. Jeremy Luban, professor of molecular medicine, biochemistry, and molecular biotechnology at UMass Chan Medical School. “The most likely explanation is that it’s more transmissible.”
But significantly, Luban said, there is no reason to think that XBB causes more severe disease than other variants.
“So far, I don’t think there’s any clear evidence that any of these variants are escaping from the protection that vaccines offer against severe disease,” he said. For people without conditions that compromise their immune system, he said, “as best as we can tell so far, the vaccines are still protective against severe disease.”
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.
A larger concern than the rising presence of any particular variant is the fact that so many different variants are circulating at the same time, said Dr. Andrew Pekosz, a professor of molecular microbiology and immunology at the Johns Hopkins University Bloomberg School of Public Health. The US Centers for Disease Control and Prevention listed 17 variants circulating in the country as of Dec. 24.
“It’s like a simmering stew. You never know when something new might jump up,” Pekosz said.
The Greek-letter-named variants first entered the common lexicon when Delta arrived in the United States in the summer of 2021, dashing hopes that the coronavirus would die down. Then Omicron stormed in around Thanksgiving of that year, a variant so transmissible that it became dominant within days. Since then, Omicron has stuck around, but shape-shifted into numerous subvariants with alphabet-soup names like BA.5 and BQ.1.1
XBB, another offshoot of Omicron, was first noted by the World Health Organization in early October and showed up in the United States around mid-November. As of last week, it accounted for 18 percent of cases nationwide.
Now, XBB has spawned its own subvariants, XBB.1 and XBB.1.5.
While XBB does not seem to cause a lot of severe illness or death, little is known about the effects of XBB.1 and XBB.1.5, which have been detected in the Northeast, said Rajendram Rajnarayanan, assistant dean of research at the New York Institute of Technology, Jonesboro, Ark., campus.
“Most of the data we have is from either the UK or other countries because they have waves before us,” Rajnarayanan said. The exception is XBB.1.5, which originated in the United States and for which there is no international data.
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.”
COVID-19 hospitalizations are increasing in Massachusetts as well. The number has been rising steadily since Thanksgiving, reaching 914 on Dec. 19. Luban, of UMass, said that many factors contribute to that trend, including the spread of illness during holiday gatherings at a time when most people have stopped wearing masks.
Also, when many more people get sick, the small proportion who need hospitalization can add up to a large number, he said.
“It’s likely that the increasing rates of hospitalization that we’re seeing are due to a combination of social factors and the fact that the virus is more transmissible,” Luban said.
The arrival of XBB is part of a process that has been going on since the pandemic erupted nearly three years ago, Luban said. The coronavirus evolves in ways that enable it to evade the defenses that people develop through vaccination or infection. In recent months, the virus has been spewing out a number of variants that got little traction, but with XBB the virus seems to have a hit upon a formula that spreads especially well.
Although there are no signs so far that XBB causes severe illness, Luban cautioned: “We have to remain vigilant about the possibility that there may be something else going on.” Any variant may develop the ability to overcome the part of the immune system that protects from severe disease, he said.
As always, the experts urged 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.
“The biggest thing that is going to be driving hospitalizations right now is individuals that have waning immunity,” said Pekosz, of Johns Hopkins. “You really need to have a recent booster, ideally the bivalent booster, because that’s what’s going to give you more protection, especially from hospitalization.”
So far, only 11.8 percent of Boston residents have received Omicron-specific bivalent boosters, according to the Boston Public Health Commission.
And while vaccination won’t prevent every infection, wearing a high-quality mask makes a big difference in blocking transmission. Rajnarayanan, of the New York Institute of Technology, said he was “heartened” to learn that Boston schools are considering enacting a temporary mask mandate.
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