New COVID variants BQ.1 and BQ.1.1 are gaining traction in the U.S. at a “troublesome” rate, according to the White House’s top medical advisor Dr. Anthony Fauci.

In the week ending Oct. 15, BQ.1 and its offspring BQ.1.1 were closing in on the position of second most widely circulating COVID variant in the United States, according to projections from the Centers for Disease Control and Prevention (CDC).

On Friday, Fauci sounded the alarm on the new variants, which accounted for an estimated 11.4% of new COVID cases in the United States as of Oct. 15—double the proportion of infections they caused a week earlier.

“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 said in an interview with CBS News.

Alongside BQ.1 and BQ.1.1’s rapid infection rates, he voiced concern about the latter’s apparent ability to evade antibody treatments being used for patients at risk of developing severe COVID symptoms.

“That's the reason why people are concerned about BQ.1.1, for the double reason of its doubling time and the fact that it seems to elude important monoclonal antibodies,” Fauci told CBS.

The BA.5 variant is currently still dominant in the U.S., accounting for around 68% of cases, while BA.4.6—which accounted for 12.2% of cases as of Oct. 15—is the second most common variant.

However, BQ.1 and BQ.1.1 have rapidly become more widespread in the U.S. in recent weeks, while BA.5 has been responsible for a smaller proportion of cases.

Despite the rise of the new variants, the latest data from the Centers for Disease Control and Prevention (CDC) paints a somewhat encouraging picture, with hospitalizations and new cases of the virus steadily declining over the past month.

Some epidemiologists and public health experts have warned, however, that Americans should brace for a resurgence of COVID infections in the coming fall and winter months—particularly with the Omicron variant continuing to evolve. Fauci backed those warnings during his interview with CBS.