Officials are hopeful the US is getting closer to the end of the pandemic. But there are road bumps ahead - CNN

"I am worried about this variant -- the B.1.1.7 variant (first found in the UK)," said Dr.

About 1,700 cases of variant strains first spotted in the UK, South Africa and Brazil have been reported in the US, according to the Centers for Disease Control and Prevention.

Rochelle Walensky said the US has ramped up sequencing to find variants and will keep doing so.

The vast majority of variant cases so far involve the highly contagious B.1.1.7 variant first detected in the UK.

"Managing the epidemic in the next four months depends critically on scaling up vaccination, trying to increase the fraction of adults willing to be vaccinated above three-quarters, and strongly encouraging continued mask use and avoiding situations where transmission is likely, such as indoor dining, going to bars, or indoor gatherings with individuals outside the household," the team wrote.

The American Medical Association, the American Hospital Association and the American Nurses Association also pleaded for Americans to help quash the pandemic.

"With new, more contagious variants of the virus circulating throughout the U.S., now is not the time to let your guard down and scale back on the measures that we know will work to prevent further illness and deaths -- wearing masks, practicing physical distancing, and washing hands," a joint statement said.

Why we could be wearing masks next winter, too

Some Americans have discovered an unexpected perk to wearing masks in the winter -- they protect against brutally cold air, not just against coronavirus.

And Americans might be wearing them next winter, when some health experts say Covid-19 might flare up again.

Fauci hopes that doesn't happen, he said, adding it's "possible" people may be wearing masks in 2022.

That clearly, really insulated the Southeast Asian countries from some of the worst of this, understanding the importance of wearing masks."

For those who have already been vaccinated, health experts say you should keep wearing a mask.