Amid the coronavirus pandemic, the use of face or cloth masks in the United States is only recommended, not enforced by the national government. Nevertheless, many states have already made their use mandatory, following studies that have highlighted their role in preventing the spread of the virus.

But that’s not enough, according to more than 100 prominent health officials, who have signed an open letter urging all governors to make wearing cloth masks in public mandatory. The letter cites a “preponderance of evidence” that the masks greatly reduce the transmissibility of COVID-19.

The letter was written by Jeremy Howard from the University of San Francisco who, along with a team of other researchers, has studied the efficacy of masks at length. Howard also wrote an opinion piece in USA Today about the letter’s recommendation.

“The preponderance of evidence, in both laboratory and clinical settings, indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets,” the letter notes. “The decreased transmissibility could substantially reduce the death toll.”

The letter said that “widespread” public mask use, alongside “other measures”, could bring down the virus’ reproduction number and halt the growth of the pandemic. By now, those “other measures” should be familiar to most people: proper hand hygiene, social distancing, testing, and contact tracing.

The co-signers of the letter include a long list of noted academics, researchers, and medical professionals, including Nobel Prize winners, medical school professors, and editors of prestigious science and medical journals. Alongside Howard, the other organizing signatory was Dr. Vincent Rajkumar.

“We ask that government officials require cloth masks to be worn in all public places, such as stores, transportation systems, and public buildings as soon as possible,” the open letter says. “This action will prevent people who are infectious from unknowingly spreading the disease.”

At the same time, the signatories asked business leaders who offer products and services to the public to require their employees and customers to wear masks whether or not it is mandated by local law. This will greatly increase the rate of mask-wearing, they argued.

Simple cloth masks appear to stop over 90% of the droplets that transmit the virus. This is crucial because research suggests that nearly half of infections are spread by people who don’t even show any symptoms, and thus they probably don’t know they’re carriers. They can go out and inadvertently spread the virus.

Cloth masks provide some protection to the wearer. But the primary role of mask-wearing is to stop people from infecting others. It also reduces the viral load if an infection does occur, which likely cuts the risk of a severe illness or death. But to be truly effective, they need to be worn by almost everyone.