As winter inches closer, Dr. Fauci is sounding the alarm about a pair of "pretty troublesome" Covid variants.

The two descendants of omicron's BA.5 subvariant, called BQ.1 and BQ.1.1, both have dangerous "qualities or characteristics that could evade some of the interventions we have," Fauci told CBS News on Friday. The two sublineages are responsible for more than 10% of all current U.S. cases, according to the latest Centers for Disease Control and Prevention data — just one week after they weren't even significant enough to list.

There's no guarantee that the two strains will eventually overtake BA.5 as the dominant variant in the country. But if they do spread rapidly, they could threaten to reverse a nationwide decline in Covid cases, hospitalizations and deaths, leading Fauci to urge Americans against letting their guard down.

"As much as you want to feel good about the fact that cases are down, hospitalizations are down, we don't want to declare victory too prematurely," Fauci, the president's chief medical adviser, said. "And that's the reason why we've got to keep our eye out on these emerging variants." 

BA.5 still makes up nearly 68% of all current U.S. cases. The new strains are responsible for almost 20% of cases in a region that includes the New York City metropolitan area, which many experts consider the area a bellwether for Covid waves — due to its dense population and high volume of incoming international travelers.

Fauci said the strains have a concerning "doubling time," referring to how quickly they spread. The U.K. Health Security Agency reported earlier this month that the strains even demonstrate a growth advantage over BA.5, which was considered the most contagious Covid strain to date.

BQ.1.1 is also particularly adept at dodging the protective antibodies you get from prior infection or vaccination, Fauci noted.

Scientists from Peking University in China published a study earlier this month that found BQ.1.1's ability to evade antibodies was "far exceeding" that of BA.5. The study, which has yet to be peer-reviewed, also found that BQ.1.1 was capable of evading immunity from a previous BA.5 infection and some antibody drugs, including Bebtelovimab and Evusheld.

Still, the fact that both strains come from BA.5 means the new omicron-specific booster shots likely provide some protection against them. Early clinical trial data suggests that the boosters work well: Pfizer's new shot generated a strong immune response against both omicron's BA.4 and BA.5 subvariants, the company reported last week.

"The somewhat encouraging news is that it's a BA.5 sublineage, so there are almost certainly going to be some cross-protection that you can boost up," Fauci said.

You can also protect yourself against the virus by wearing a mask in high-risk areas or situations. The CDC recommends masking indoors if you're in a county with a substantial or high-level spread of Covid.

Fauci, during an event with the USC Annenberg Center for Health Journalism earlier this month, said "it's entirely conceivable" that the U.S. will see enough of an uptick to encourage indoor masking nationwide again.

"I hope not," he added.

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