The more the virus has the opportunity to spread, the greater the risk that it will mutate and more variants will appear, as have the variants first identified in the United Kingdom, South Africa and Brazil.

"And if the vaccines aren't as effective against some of these variants, then we could see the gains that we're so excited about right now, we could see those reversed in a very short amount of time."

Now, some people are even recommending wearing two masks at once.

Here's what you should know about masks.

Here's where masks are required

President Joe Biden has called on everyone to wear a mask when out in public, and he has made it mandatory on federal property. Masks are required on public transportation and in airports and transit stations.

It's important to note that children younger than 2 should not wear a mask, according to the CDC.

ASTM International, an international technical standards organization, and the National Personal Protective Technology Laboratory are working on standards on filter efficiency, sizing and fit, cleaning and recommended period of use or reuse.

What type of mask should you wear?

Surgical masks have three layers and generally offer more protection than cloth masks.

Cloth masks have the advantage of being washable and reusable. Be sure that the mask has at least two layers -- the more layers, the better.

"If you're wearing a cloth mask, it should be a multi-layered mask so that you have several layers of potential protection for a single mask," CDC Director Dr. Rochelle Walensky told CNN's Anderson Cooper and Sanjay Gupta.

Aside from their scarcity, N95 masks are "very hard to tolerate when you wear them for long periods of time," Walensky said.

"I worry that if we suggest or require that people wear an N95, they won't wear them all the time. They're very hard to breathe in, when you wear them properly," she said.

Should you wear two masks?

Fauci, director of the National Institute of Allergy and Infectious diseases, recently has been sporting two masks, one on top of the other. The CDC hasn't issued any guidance on doubling up, but Fauci said that's because it doesn't yet have scientific data backing up the effectiveness.

In the meantime, Fauci said, "there are many people who take the commonsense approach. If you're talking about a physical barrier, and as the CDC recommends, you want at least two layers within the mask as a physical barrier, and you feel maybe more of a physical barrier would be better."

The main thing to know is that wearing a mask, any mask, and keeping your distance from others will help the country out of this pandemic.

"The most important thing is that everybody should be wearing a mask," Fauci told Anderson and Gupta.