While CDC lowered its projections of XBB.1.5's prevalence in the United States, experts say the subvariant is so "crazy infectious" that "everyone in the country is at risk for infection now."

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CDC revises its XBB.1.5 estimates

In late December, CDC's COVID-19 variant dashboard featured an emerging omicron subvariant that many experts believed had the potential to trigger a new surge of COVID-19 infections.

For the week ending on Dec. 31, CDC estimates initially projected that the XBB.1.5 strain was responsible for 40.5% of confirmed COVID-19 cases in the United States—a significant increase from 20% for the week ending on Dec. 24. 

However, CDC on Friday revised their estimates, lowering the subvariant's prevalence from 40.5% of U.S. cases to 18.3% of cases for the week ending Dec. 31.

For the week ending Jan. 7, CDC reported that XBB.1.5 was responsible for 27.6% of COVID-19 cases.

"We're seeing sustained increases of COVID infections across the nation," said White House COVID-19 response coordinator Ashish Jha.

Currently, XBB.1.5 is the only omicron subvariant that is still growing in the United States. According to Sam Scarpino, who has been tracking new variants at Northeastern University, XBB.1.5 "shot up like a rocket."

"This variant has displaced other variants in a way that we've never seen before. That's kind of alarming," Scarpino said.

XBB.1.5 is 'crazy infectious'

In a Twitter thread, Jha noted that it is still unclear if the XBB.1.5 variant is more severe than its predecessors—but many experts believe the strain is unlikely to trigger a significant increase in hospitalizations and deaths.

Still, Maria Van Kerkhove, the World Health Organization's technical lead on COVID-19 said XBB.1.5 "is the most transmissible subvariant that has been detected yet. We are concerned about its growth advantage."

Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center who has been studying variants, said, "XBB.1.5 has gotten a specific mutation that enables it to maintain antibody escape properties while also giving it an advantage for spreading through the population," which will likely lead to an increase in COVID-19 infections.

"The question is not whether it's going to cause a surge. It almost certainly will. The question is: How big is the surge going to be?" Scarpino said.

According to Paula Cannon, a virologist at the University of Southern California, "Essentially, everyone in the country is at risk for infection now, even if they're super careful, up to date on vaccines, or have caught it before," USA Today reports.

Health officials encourage COVID-19 prevention measures

Meanwhile, many experts are encouraging individuals to implement COVID-19 prevention measures. "We do expect further waves of infection around the world, but that doesn't have to translate into further waves of death because our countermeasures continue to work," Kerkhove said.

"What is clearer now, compared to even a year ago, is that we can really blunt the worst of it by doing the things that we know work," Jha said.

"The vaccines and the booster are still highly effective against hospitalizations and deaths," said Ali Mokdad, a professor of health metrics sciences at the Institute for Health Metrics and Evaluation at the University of Washington.

In addition, experts recommend taking other precautions, including avoiding crowded, poorly ventilated areas, testing before social events, and wearing a mask.

Ziyad Al-Aly, chief of research and development at the VA St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis, said individuals should be doing everything they can to avoid a COVID-19 infection.

According to Al-Aly, there is still a lot that remains unknown about XBB.1.5—and his research suggests that every COVID-19 infection poses a risk of serious infection and long COVID. "Reinfection buys you additional risk," he said.

"It is a time not to let your guard down," said Tina Tan, an infectious disease specialist at Northwestern University. (Scribner, Axios, 1/7; Weintraub, USA Today, 1/9; Bean, Becker's Hospital Review, 1/6; Irfan, Vox, 1/6; Stein, "Shots," NPR, 1/6)