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The Covid Omicron XBB.1.5 variant is rapidly becoming dominant in the U.S. because it is highly immune evasive and appears more effective at binding to cells than related subvariants, scientists say.

XBB.1.5 now represents about 41% of new cases nationwide in the U.S., nearly doubling in prevalence over the past week, according to the data published Friday by the Centers for Disease Control and Prevention. The subvariant more than doubled as a share of cases every week through Dec. 24. In the past week, it nearly doubled from 21.7% prevalence.

Scientists and public health officials have been closely monitoring the XBB subvariant family for months because the strains have many mutations that could render the Covid-19 vaccines, including the omicron boosters, less effective and cause even more breakthrough infections.

XBB was first identified in India in August. It quickly become dominant there, as well as in Singapore. It has since evolved into a family of subvariants including XBB.1 and XBB.1.5.

Andrew Pekosz, a virologist at Johns Hopkins University, said XBB.1.5 is different from its family members because it has an additional mutation that makes it bind to cells better.

“The virus needs needs to bind tightly to cells to be more efficient at getting in and that could help the virus be a little bit more efficient at infecting people,” Pekosz said.

Yunlong Richard Cao, a scientist and assistant professor at Peking University, published data on Twitter Tuesday that indicated XBB.1.5 not only evades protective antibodies as effectively as the XBB.1 variant, which was highly immune evasive, but also is better at binding to cells through a key receptor.

Scientists at Columbia University, in a study published earlier this month in the journal Cell, warned that the rise of subvariants such as XBB could “further compromise the efficacy of current COVID-19 vaccines and result in a surge of breakthrough infections as well as re-infections.”

The XBB subvariants are also resistant to Evusheld, an antibody cocktail that many people with weak immune systems rely on for protection against Covid infection because they don’t mount a strong response to the vaccines.

The scientists described the resistance of the XBB subvariants to antibodies from vaccination and infection as “alarming.” The XBB subvariants were even more effective at dodging protection from the omicron boosters than the BQ subvariants, which are also highly immune evasive, the scientists found.

Dr. David Ho, an author on the Columbia study, agreed with the other scientists that XBB.1.5 probably has a growth advantage because it binds better to cells than its XBB relatives. Ho also said XBB.1.5 is about as immune evasive as XBB and XBB.1, which were two of the subvariants most resistant to protective antibodies from infection and vaccination so far.

Dr. Anthony Fauci, who is leaving his role as White House chief medical advisor, has previously said that the XBB subvariants reduce the protection the boosters provide against infection “multifold.”

“You could expect some protection, but not the optimal protection,” Fauci told reporters during a White House briefing in November.

Fauci said he was encouraged by the case of Singapore, which had a major surge of infections from XBB but did not see hospitalizations rise at the the same rate. Pekosz said XBB.1.5, in combination with holiday travel, could cause cases to rise in the U.S. But he said the boosters appear to preventing severe disease.

“It does look like the vaccine, the bivalent booster is providing continued protection against hospitalization with these variants,” Pekosz said. “It really emphasizes the need to get a booster particularly into vulnerable populations to provide continued protection from severe disease with these new variants.”

Health officials in the U.S. have repeatedly called on the elderly in particular to make sure they are up to date on their vaccines and get treated with the antiviral Paxlovid if they have a breakthrough infection.

The new omicron Covid boosters probably aren’t very effective at preventing Covid infections and mild illness, but they will likely help keep the elderly and other vulnerable groups out of the hospital this winter, experts say.

The Centers for Disease Control and Prevention, in a real-world study published this week, found the boosters are less than 50% effective against mild illness across almost all adult age groups when compared to people who are unvaccinated.

For seniors, the booster was 19% effective at preventing mild illness when administered as their fourth dose, compared to the unvaccinated. It was 23% effective against mild illness when given as their fifth dose.

Though the vaccine’s effectiveness against mild illness was low, people who received the boosters were better off than those who did not. The booster increased people’s protection against mild illness by 28% to 56% compared to those who only received the old shots, depending on age and when they received their last dose.

The Food and Drug Administration authorized the boosters in late August with the goal of restoring the high levels of protection the vaccines demonstrated in late 2020 and early 2021. At that time, the shots were more than 90% effective against infection. But the first real-world data from the CDC indicates that the boosters aren’t meeting those high expectations.

“The boosters give you some additional protection but it’s not that strong, and you shouldn’t rely on it as your sole protective device against infection,” said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College.

Moore said people at higher risk from Covid have every reason to get a booster since it modestly increases protection. But he said common sense measures such as masking and avoiding large crowds remain important tools for vulnerable groups since the boosters aren’t highly effective against infection.

The CDC study looked at more than 360,000 adults with healthy immune systems who tested for Covid at retail pharmacies from September to November when omicron BA.5 was dominant. The participants received either the booster, got two or more doses of the old shots or they were unvaccinated. It then compared those who tested positive for Covid with those who did not.

The study did not evaluate how well the boosters performed against severe disease, so it’s still unclear whether they will provide better protection against hospitalization than the old shots. The CDC in a statement said it will provide data on more severe outcomes when it becomes available.

Andrew Pekosz, a virologist at Johns Hopkins University, said the fact that the shots are providing some protection against infection in an era of highly immune evasive omicron subvariants is a good sign that they will provide strong protection against hospitalization. The vaccines have always performed better against severe disease than mild illness, he said.

“It’s better than nothing. Certainly, it doesn’t sort of show that the protection is incredibly high against infection,” Pekosz said. “I would expect that you would then see even greater protection from hospitalization or death.”

Dr. Paul Offit, a member of the FDA’s vaccine advisory committee, said trying to prevent mild illness is not a viable public health strategy because the antibodies that block infection simply wane over time.

“Protection against mild disease just isn’t that good in the omicron subvariant era. The goal is protecting against severe disease,” said Offit, an infectious disease expert at Children’s Hospital of Philadelphia who helped develop the rotavirus vaccine.

Dr. Celine Gounder, a senior public health fellow at the Kaiser Family Foundation, said she’s not alarmed by the data. Reducing risk by even a modest amount at the individual level can have a significant positive effect on public health at the population level.

“If you can reduce risk among the elderly by even 30%, even 20%, that is significant when 90% of the COVID deaths are occurring in that group,” Gounder said. “For me, what’s really gonna matter is are you keeping that 65 year old out of the hospital.”

The boosters, called bivalent vaccines, target both omicron BA.5 and the original Covid strain that first emerged in Wuhan, China in 2019. The original shots, called monovalent vaccines, only include the first Covid strain.

It’s still unclear how the boosters will perform against more immune evasive omicron subvariants, such as BQ.1 and BQ.1.1, which are now dominant in the U.S. Pfizer and Moderna last week said early clinical trial data shows the boosters induce an immune response against these subvariants.

About 11% of those eligible for the new booster, or 35 million people, have received it so far, according to CDC data. About 30% of seniors have received the shot.