A new version of omicron has taken hold in the US, according to the latest data from the Centers for Disease Control and Prevention. The omicron sub-variant, called XBB.1.5, has raised concerns about another possible wave of covid cases after the busy holiday travel season.

The CDC projected on Friday that about 40% of confirmed covid cases in the US are caused by the XBB.1.5 strain, up from 20% a week ago. In the Northeast, about 75% of confirmed cases are reported to be XBB.1.5.

It’s still unclear where this version of omicron came from, but it seems to be spreading fast here. There is no indication that it causes more severe illness than any other omicron virus, Dr. Barbara Mahon, director of the CDC’s Division of Coronavirus and Other Respiratory Viruses, told NBC News.

While Covid hospitalizations overall are rising across the country, areas like the Northeast that have seen high levels of the new variant have not seen a disproportionate increase in hospitalizations, Mahon said.

“We are seeing that hospitalizations have increased overall across the country,” he said. «They don’t seem to be getting any better in the areas that have more XBB.1.5.»

The seven-day average of daily Covid hospitalizations reached 42,140 on Friday, an increase of 4.2% from the previous two weeks, according to a tally by NBC News. The seven-day average of daily intensive care unit admissions also rose to 5,125 per day, an increase of more than 9% from two weeks ago.

Much is still unknown about the latter subvariant, including whether it is more contagious than other forms of omicron, Mahon said.

Other scientists worry that XB.1.5 will be even better at getting around the antibodies we’ve developed from the Covid vaccines and previous infection from the many different types of omicron that have spread since last December, including the original BA.1. and the most recent. Subvariants BQ.1.1 and BQ.1.

The XBB.1.5 is a relative of the omicron XBB variantwhich is a recombinant of the omicron subvariants BA.2.10.1 and BA.2.75.

Combined, XBB and XBB.1.5 account for 44% of cases in the US, displacing other omicron versions.

XBB has been found in at least 70 countries, according to the World Health Organization, leading to infection surges in parts of Asia, including India and Singapore, in October.

Studies done in the lab have found that XBB is able to evade antibodies from previous Covid infections or vaccines, meaning that being exposed to the virus would mean someone is more likely to get sick or reinfected and show symptoms.

“It is clear that there are immune-evasive properties of XBB,” said Dr. Isaach Bogoch, an infectious disease physician and epidemiologist at the University of Toronto. “That has been shown both in laboratory studies and seen clinically in cases and hospitalizations.”

Given the high level of immunity in the US population, whether through infection, vaccination, or both, Bogoch and others hope that even if cases begin to rise significantly, there will not be a dramatic increase in hospitalizations or deaths as seen in previous waves. .

Antibody studies don’t tell the whole story. Other parts of the immune system can protect against the virus and Covid vaccines should remain effective in preventing severe illness and death from the virus, the evidence suggests.

«We certainly could have a wave, but it’s much less likely to be as deadly or overwhelming for a health care system compared to previous waves before we had this degree of hybrid immunity,» Bogoch said.

Do Covid vaccines work against XBB.1.5?

As encouraging signs, Rick Bright, an American immunologist and former director of the Biomedical Advanced Research and Development Authority, or BARDA, pointed to Singapore’s experience with XBB.

There was an increase in cases, but «we didn’t see corresponding major increases in hospitalizations and deaths,» Bright said.

“We think it is because a larger population of people in Singapore have been vaccinated with the latest vaccines and boosters,” he said.

That could be a problem in the United States, unfortunately.

People over the age of 65 are the most vulnerable to any form of the Covid virus. However, only 37.5% of that age group have received the most recent omicron booster, according to the latest CDC data.

Experts agree that the most important thing is to get a booster shot with the new bivalent vaccines to bolster your immune system against the new variants.

«It’s not 2020, but people still need to take this seriously and protect themselves,» Mahon said, adding that getting the bivalent booster is especially important for people older than 65, a high-risk group that has seen uptake of reinforcement quite low recently.

A study published last week in the New England Journal of Medicine found that new Covid boosters from Moderna and Pfizer-BioNTech bolstered antibody responses to many Omicron subvariants, including the XBB variant.

While the new booster, called bivalent because it targets the omicron BA.4 and BA.5 strains as well as the original coronavirus, isn’t perfect, it does offer additional protection than was seen in previously available original or monovalent boosters, said Mehul Suthar, an associate professor at the Emory Vaccine Center at Emory University and author of the report.

«With the monovalent boosters, your neutralizing antibodies just aren’t as potent against the variants, but the bivalent booster makes sure it’s a little better,» he said. «Not surprising, but better, indicating that the bivalent boosters are working as they should.»

The new XBB.1.5 variant was not studied in the report, but Suthar predicts that its evasive immune properties will be in a similar range to XBB. He hopes that the bivalent boost will also strengthen protection against the latest version.