Estimated cases of Omicron variants BQ.1 and offspring BQ.1.1—dubbed Typhon and Cerberus respectively by some experts in the Twitterverse—rose from about 12% nationally last week to nearly 17% this week, the U.S. Centers for Disease Control and Prevention announced Friday. Combined, they’ve overtaken fellow Omicron spawn BA.4.6 as the projected No. 2 variant in the U.S.

Once globally dominant BA.5, a parent variant to both, continues to lead, but with a dwindling proportion of estimated cases. It sits at an estimated 62% of cases, down from 70% the week prior.

The nature of the beast

Along with XBB, a combination of two Omicron strains spiking in Singapore, BQ.1.1 is thought to be the most immune-evasive new variant yet—and one that clearly has the ability to push back on BA.5, still dominating the U.S., though its levels are dropping steadily.

“When you get variants like that, you look at what their rate of increase is as a relative proportion of the variants, and this has a pretty troublesome doubling time,” Fauci, speaking of the BQ variants, told CBS News on Friday.

BQ.1.1’s extreme immune evasiveness and transmissibility “sets it up to be the principal driver of the next U.S. wave in the weeks ahead,”  Dr. Eric Topol, a professor of molecular medicine at Scripps Research and founder and director of the Scripps Research Translational Institute, tweeted last week.

Last week, he told Fortune that scientists won’t know to what extent it challenges vaccines, if it does, until it reaches 30% to 50% of cases somewhere.

“It’s not going to wipe out vaccine efficacy, but it could put a dent in protection against hospitalizations and death,” he said.

BQ.1.1 is already known to escape antibody immunity, rendering useless monoclonal antibody treatments used in high-risk individuals with COVID. According to a study last month out of Peking University’s Biomedical Pioneering Innovation Center in China, BQ.1.1 escapes immunity from bebtelovimab, the last monoclonal antibody drug effective on all variants, as well as Evusheld, which works on some. Along with variants BA.1 and XBB, BQ.1.1 could lead to more severe symptoms, the authors wrote.

The good news, if there is any, about BQ.1 and BQ.1.1 is that new Omicron boosters will “almost certainly” provide “some” protection against them because they were designed to tackle close relative BA.5, Fauci told CBS last week.

The backstory

Until last week, the CDC had listed BQ variants under BA.5, the parent lineage of both. They appeared to grow from 0% to nearly 20% of regional cases overnight. But CDC data, updated retroactively, shows that the duo has been slowly growing since mid-September. It constituted a projected 5.7% of cases the week before last, and a little less than 3% of cases the week before that.

BQ.1 and BQ.1.1 were first detected in mid-July, according to an Oct. 5 risk assessment from the Ontario public health department. It rates the risk level of increased transmissibility, reinfection, and reduced vaccination effectiveness against infection as high, with a high degree of uncertainty.

Many experts, including those at the Institute for Health Metrics and Evaluation at the University of Washington, as well as Fauci, predict a coming wave of infections that will swell this month and peak in late December or early January.

As of Wednesday, weekly U.S. COVID cases sat at nearly 261,000, an average of 37, 285 a day. That’s up from an average of 37,000 cases a day as of last week. This week the CDC stopped reporting seven-day averages of newly reported cases and is only reporting weekly totals.

With testing at all-time lows, it’s widely accepted that case numbers reported to the agency, and thus reported by it, pale in comparison to the actual number of cases in the U.S.