The Centers for Disease Control and Prevention released new research on Wednesday that found wearing a cloth mask over a surgical mask offers more protection against the coronavirus, as does tying knots on the ear loops of surgical masks. Those findings prompted new guidance on how to improve mask fit at a time of concern over fast-spreading variants of the virus.

For optimal protection, the CDC says to make sure the mask fits snugly against your face and to choose a mask with at least two layers.

The Centers for Disease Control and Prevention released new research on Wednesday that found wearing a cloth mask over a medical mask is one way to reduce exposure to aerosol droplets that can transmit the coronavirus.

In laboratory testing, researchers simulated coughs and breathing and tested how well different masks worked to block aerosol particles — comparing no mask, a cloth mask or a surgical mask. They also tested two methods to optimize the fit of cloth and medical masks: wearing a cloth mask over a surgical mask, and tying knots on the ear loops of surgical masks and then tucking in and flattening the extra material against the face.

Both methods produced substantially improved protection against transmission of and exposure to infectious aerosols.

"In the study, wearing any type of mask performed significantly better than not wearing a mask," said CDC Director Rochelle Walensky in a briefing from the White House COVID-19 Response Team on Wednesday.

"And well-fitting masks provided the greatest performance at both blocking emitted aerosols and exposure of aerosols to the receiver. In the breathing experiment, having both the source and the receiver wear masks modified to fit better reduced the receiver's exposure by more than 95%, compared to no mask at all," she said.

The CDC compared the protection offered by (from left) an unknotted medical procedure mask, a cloth mask covering medical procedure mask, and medical procedure mask that is knotted and tucked.

Walensky said the laboratory findings underscore the importance of wearing a mask correctly and ensuring it fits snugly over your nose and mouth. The new information does not alter the CDC's guidance on who should wear a mask and when.

As of Feb. 2, masks are now required on planes, buses, trains and other public transportation traveling into, within or out of the U.S., as well as in U.S. transit hubs such as airports and stations.

The CDC's guidance on improving mask fit now recommends:

* Choosing a mask with a nose wire, which prevents air from leaking out along the top

* Using a mask fitter or brace over a disposable or cloth mask, to prevent air from leaking out. These small, reusable devices cinch a cloth or medical mask to create a tighter fit and improve mask performance, Walensky said.

* Make sure your mask fits snugly over your nose, mouth and chin: "If the mask has a good fit, you will feel warm air come through the front of the mask and may be able to see the mask material move in and out with each breath," the CDC says.

* Add layers of material – either by using cloth mask with multiple layers of fabric or by wearing a disposable mask under a cloth mask.

But double-masking isn't recommended for all masks. Don't combine two disposable masks, the CDC says: "Disposable masks are not designed to fit tightly and wearing more than one will not improve fit."

And don't layer another mask on top of a KN95, either. That mask should be used alone, the CDC says.