How we know that masks, even the cloth ones, reduce the spread of COVID-19

Two years into the COVID-19 pandemic, the public debate on masks is as heated as ever — especially in Philadelphia, which earned national attention for once again requiring masks in indoor public spaces and then undoing that rule days later.

Yet in the realm of science, there is plenty of evidence to support using the face coverings. Researchers who study airborne transmission of viruses say there is no question that masks — even the cloth variety — reduce the spread of the coronavirus.

Exactly how much depends on the type of material, how well the mask fits, and how many virus particles an infected person is shedding, among other factors. No mask is bulletproof, but that’s not a reason to reject them entirely. No infection-control measure is absolute, short of complete isolation.

That’s why public health officials have stressed multiple layers of protection: masks, social distancing, and, above all, vaccines. If one layer fails, another may do the trick.