The Duke researchers found that neck fleeces made of polyester spandex could actually increase the rate of droplet transmission during normal speech compared to no mask at all.

Instead of blocking large droplets, neck fleeces appear to disperse these droplets into smaller, more numerous particles. Since small droplets tend to linger in the air for longer, the researchers suggested neck fleeces may be "counterproductive." But that likely depends on their material and how many layers of fabric they have. 

But How You Wear Your Mask Matters, too.

The protectiveness of a mask — including N95 and surgical masks — declines considerably when there is a gap between the mask and the skin.

"It's about the seal of the mask," Asfour said. "You have to make sure there's no air leak."

Even so, research has shown that wearing masks improperly or sporadically could still reduce transmission. In a July editorial published in the Journal of the American Medical Association, CDC Director Robert Redfield predicted that the universal adoption of face masks could bring the US's outbreak under control in as little as four weeks based on case numbers and transmission rates at that time.