• XBB.1.5 made up 27.6% of sequenced Covid cases nationally for the week ending Jan. 7 compared to 18.3% for the week end Dec. 31.
• The World Health Organization has described XBB.1.5 as the most transmissible version of Covid yet.
• Scientists say XBB.1.5 has a mutation that makes it bind to human cells better, which may make it better at infecting people than other variants.

The Centers for Disease Control and Prevention on Friday revised downward its estimate of how much the omicron XBB.1.5 subvariant is circulating across the U.S., though it continues to spread at a faster pace than other versions of Covid-19.

XBB.1.5 made up 27.6% of sequenced Covid cases nationally for the week ending Jan. 7 compared with 18.3% for the week end Dec. 31. The CDC previously reported that XBB.1.5 made up about 41% of sequenced cases for the week ending Dec. 31, more than any other variant.

Although the agency has revised its estimate downward, XBB.1.5 remains the only omicron subvariant showing significant growth in the U.S. right now. It is second only to omicron BQ.1.1, which currently makes up 34% of sequenced Covid cases in the U.S.

XBB.1.5 makes up more than 70% of sequenced cases in the northeastern U.S., which is often a bellwether for the rest of the country.

The World Health Organization has described XBB.1.5 as the most transmissible version of Covid yet. Scientists say XBB.1.5 has a mutation that makes it bind to human cells better, which may make it better at infecting people than other variants.

Dr. Ashish Jha, who heads the White House Covid taskforce, said in a series of Twitter posts Wednesday that the XBB.1.5 subvariant is probably more immune evasive and may also be inherently more contagious because it binds more tightly to human cells.

Jha said It's unclear whether XBB.1.5 is more dangerous than past variants. But Dr. Robert Califf, who heads the Food and Drug Administration, noted in a series of Twitter posts Wednesday that for now, cases are increasing with no evidence of increased severity of illness.

Jha warned that people who last had a Covid shot before September or who had an infection before July probably do not have strong protection against XBB.1.5. Seniors who are not up to date on their shots are increasingly vulnerable to serious illness, Jha said.

U.S. health officials should have more data soon on how much protection the omicron boosters provide against XBB.1.5., Jha said. Califf said the boosters should provide some protection against the subvariant based on studies that looked at other subvariants in the same family, XBB and XBB.1.

"It is highly likely that the current bivalent vaccines provide some protection against XBB, especially in the prevention of serious illness and death," Califf wrote on Twitter.

However, scientists at Columbia University, in a recent study, noted that variants in the XBB family pose a serious threat to the omicron boosters.

Weekly Covid cases have increased by about 16% to 470,699 over the past week, according to CDC data. Average daily hospital admissions have increased 16% to more than 6,500 over the past week, according to the data. Weekly deaths have also increased 8% over the week prior to more than 2,700.