What to Know About the New XBB.1.5 Variant

A new variant of the virus that causes COVID-19 is spreading in the U.S., raising concerns about a potential wave of infections and reinfections to start the new year.

The variant, called XBB.1.5, is a descendent of Omicron and a close relative of the XBB variant, which spread widely in Singapore and India this past fall. A December study in the journal Cell demonstrated that XBB is better at evading immune defenses gained from vaccination and prior infection, compared to other variants. This raises the risk of reinfection, a World Health Organization group warned in October—though the group said at the time that XBB does not appear to cause significantly more severe disease than previous strains.

Along with its adeptness at getting around immune blockades, XBB.1.5 appears to be highly transmissible, thanks to some key mutations picked up as the virus evolved. These tweaks are stoking concerns about a surge in cases this winter—particularly given low rates of booster uptake and relaxed disease-mitigation measures.

While there is limited research on XBB.1.5 at this point, here’s what we know so far.

How widespread is XBB.1.5 in the U.S.?

During the week ending Dec. 31, XBB.1.5 accounted for 40.5% of new sequenced COVID-19 cases in the U.S., according to data from the U.S. Centers for Disease Control and Prevention (CDC). It’s currently causing roughly 75% of new cases in the Northeast, which is often a bellwether for the rest of the country.

Nationally representative diagnostic data from Walgreens, a COVID-19 testing provider across the U.S., shows almost 40% of tests are now coming back positive, though it’s not possible to say how many of those infections were due to XBB.1.5. Hospitalizations are also starting to tick upward nationwide, according to CDC data.

Do vaccines and treatments work against XBB.1.5?

While there isn’t much data on XBB.1.5 yet, research on its relative XBB provides some clues. Research recently published in the New England Journal of Medicine (and based on a small number of people) suggests that while XBB is more immune-evasive than previous versions of the virus, people who have received the updated bivalent booster are better protected against it than those who have not. Just 15% of people in the U.S. ages 5 and older have gotten a bivalent booster, according to the CDC, which means many people are currently not as protected as they could be against the new variant.

Whether or not monoclonal antibody treatments are effective against XBB.1.5 is another concern. In the fall of 2022, federal health officials acknowledged that some monoclonal antibody therapies do not work well against newer variants, which is particularly concerning for immunocompromised people who do not respond well to vaccines. The recent Cell study found that these therapies largely did not work against XBB, which suggests the same may be true for XBB.1.5.

Will XBB.1.5 lead to a new wave of Long COVID cases?

Long COVID, the name for enduring and often-debilitating symptoms that follow a case of COVID-19, can affect anyone infected by SARS-CoV-2, including those who are vaccinated and initially had mild disease. Some data suggest that people infected by earlier Omicron variants were less likely to develop Long COVID than those who caught Delta. But if XBB.1.5 spreads widely, even a small percentage of people developing long-term complications could mean lots of new Long COVID cases.

Avoiding infection is the best way to stay healthy in both the short and long term. To do that, follow all the usual advice: stay up-to-date on vaccines and boosters, wear a high-quality mask in public indoor areas, meet with others outdoors or in well-ventilated places if possible, and consider avoiding group gatherings if infection rates are high in your area.

Write to Jamie Ducharme at jamie.ducharme@time.com.