There’s Already Another New Dominant Covid Strain. Here’s What to Know.

In the Northeast, the fast-spreading XBB.1.5 variant is causing a majority of new cases.

A woman wearing a head covering is blowing her nose.
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Coronavirus cases and hospitalizations have steadily ticked up in the United States over the past couple months. This is due, at least in part, to half a dozen subvariants that seem to be better at immune evasion and at transmission than previous virus versions.

Now, one subvariant, XBB.1.5, seems to be spreading even faster. A hybrid of two BA.2 variants and a relative of the so-called “nightmare variant,” XBB, which caused a surge in Singapore in the fall, XBB.1.5 is raising concerns about a potential wave in the United States as people return from holiday travel.

According to the Centers for Disease Control and Prevention, XBB.1.5 nearly doubled in prevalence every week in December. Just before Christmas, it accounted for 21.7 percent of new Covid cases in the United States. In an update on Dec. 30, the C.D.C. said that XBB.1.5 made up 40.5 percent of cases and had edged out the other subvariants (like BA.5, BF.7, BQ.1 and BQ.1.1, among others) that are currently responsible for the rest of the cases in the U.S. In the Northeast, XBB.1.5’s foothold is even stronger: It is causing roughly 75 percent of new cases.

Here’s what experts know so far about the new subvariant and what its mutations may mean for repeat infections, symptoms, case numbers and treatment options.

The evolution of coronavirus variants is nothing new. “Viruses and pathogens are constantly trying to adapt and escape the immune pressure that we pose to them,” said Dr. Albert Ko, a physician and epidemiologist at the Yale School of Public Health.

XBB.1.5 is slightly different from its relatives, because it not only evades protective antibodies, it also is better at binding to cells. That means healthy adults are more likely to be infected with it even after vaccination or an infection with a different variant.

But other parts of the immune system can still come to our defense, said Dr. Otto Yang, an infectious disease physician and immunology researcher at David Geffen School of Medicine at the University of California, Los Angeles.

Even if mutations in some parts of the virus prevent antibodies from blocking an infection, T cells should be able to recognize the main part of the virus spike protein, Dr. Yang said. And because T cells remember how to respond to the coronavirus based on what they’ve learned from previous encounters or vaccines, they can mount an attack quickly if you do become infected. “They are what prevent severe illness,” Dr. Yang said. “People who are up to date on their vaccines and who get treatment early with Paxlovid or with remdesivir are going to do fine, for the most part.” (Paxlovid is an oral antiviral medication, and remdesivir is an injectable one.)

Immune evasion and infection are a bigger concern for people who are immunocompromised or who do not mount a strong immune response to vaccines. That includes people who have had stem cell or solid organ transplants, people receiving cancer treatments, people with autoimmune diseases and people who need immunosuppressive medication for various medical conditions, said Dr. Alpana Waghmare, an infectious disease expert at Fred Hutchinson Cancer Center.

The antibody treatments that many immunocompromised people have relied on, like the preventive shot Evusheld and the intravenous Covid treatment bebtelovimab, are not effective against XBB.1.5 and other new subvariants. As a result, immunocompromised people may have more severe disease, but other antiviral treatments will most likely keep deaths from increasing in this group.

There is no evidence yet to suggest that people who are infected with any of the new subvariants experience new or unusual symptoms or that their illness is more severe than what we’ve seen with previous Omicron variants, Dr. Waghmare said.

The most common symptoms still include a mild runny nose, headache and sore throat. These could precede a positive Covid test, and symptoms could be mild or moderate for the duration of the illness.

The best thing most people can do to protect themselves is to be vaccinated and boosted. In particular, those who are at high risk, as well as those who have not had a booster shot or a Covid infection in the last four to six months, should make a plan to get a shot, Dr. Yang said. Some experts recommend a booster even sooner.

The bivalent booster can train your immune system to recognize the original virus from 2020 as well as the BA.5 Omicron variant. Although it remains to be seen how well antibodies from the bivalent shot may work against XBB.1.5, the T cell protection that arises from the booster should still be able to protect you against severe illness if you are infected, Dr. Yang said. In terms of vaccine effectiveness, “if we see that deaths are reduced and if serious illness and hospitalizations are reduced, even if people do get infected, that’s still a big success,” said Michael Osterholm, an epidemiologist and the director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

People who are at high risk of severe Covid-19 or have vulnerable family members should also consider wearing a mask, avoiding crowded indoor venues and asking others about Covid symptoms or exposure, particularly after gatherings. They should also take frequent Covid tests and seek antiviral medication such as Paxlovid early on if a test comes back positive, Dr. Osterholm said.