When COVID-19 began to crawl across the United States, Americans watched as it transformed the landscape of their communities: Essential stores became lined with tape marks to enforce physical distancing and social media became awash with videos – from comic to institutional – on how to wash one’s hands. Physical distancing and hand-washing are critical tools to stop the spread of COVID-19. Then on April 3, the Center for Disease Control and Prevention issued a recommendation to add one more tool: cloth face coverings.

While surgical and N95 masks should be reserved for medical personnel, the CDC advised, individuals in public spaces where physical distancing may be difficult should wear cloth face coverings to inhibit the spread of the virus. Within weeks, cities issued facial covering requirements with varying degrees of severity. San Francisco began asking residents to wear them in lines and in stores, but last month, it instituted one of the more stringent mandates, requiring individuals (with a few exceptions) to wear coverings when outside or within 30 feet of people who do not live in their households.

What changed? 

Research and interviews with several epidemiologists indicate that more than any specific piece of research on masks, it was the evolving understanding of the virus that made masks newly relevant. And, that understanding is still evolving.

“One of the biggest improvements has been our understanding of asymptomatic transmission of COVID-19,” Marie Stoner, an epidemiologist at Research Triangle International, a nonprofit research institute based in North Carolina. “This finding has been informed by more and more studies that have come out documenting asymptomatic transmissions and asymptomatic people in the community.” Stoner pointed to several studies also cited by the CDC for this finding.

In its recommendation for cloth face coverings, the CDC lists seven different studies published between mid-February and May as the basis for its new mask policy. As opposed to research on the effectiveness of cloth coverings, each study focuses on asymptomatic transmission. 

The first study, published in the New England Journal of Medicine in early March, reported that a German businessman appeared to have contracted COVID-19 from a visiting business colleague who was not showing symptoms when they were in contact before both tested positive for COVID-19. Another study cited by the CDC found that the viral load of SARS-CoV-2, the virus that causes COVID-19, in an asymptomatic patient is similar to those of symptomatic patients, “suggest[ing] the transmission potential of asymptomatic or minimally symptomatic patients.”

“Because we now know that people can transmit the virus without showing symptoms, this impacts how we must respond to prevent transmission,” Stoner said. “It is important to limit contact and wear masks because you or someone else might be positive for COVID-19 but not yet showing symptoms.”

Because the virus is thought to spread primarily through droplets produced when infected individuals cough, sneeze, or speak, masks, in theory, are meant to catch these droplets. There are also questions regarding the potential spread of the virus through smaller, aerosolized particles that would make the virus airborne.

While research shows that asymptomatic carriers can spread COVID-19, on June 8, the head of WHO’s emerging diseases and zoonosis unit, Dr. Maria Van Kerkhove, cast doubt on how often that occurs. Talking to reporters at a news conference in Geneva, she said that based on current data, asymptomatic spread “seems to be rare.” UPDATE: On Tuesday Van Kerkhove “walked back” on that assertion, the NYTimes reported.

As the body of research on asymptomatic spread continues to grow, there is, so far, little systematic study on the effectiveness of cloth coverings in inhibiting COVID-19’s spread.

Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy and a Regents Professor at the University of Minnesota, expressed concern about the lack of research regarding cloth face coverings in a special podcast episode produced by the center. He’s not against cloth coverings, but he’s waiting for the evidence to catch up to the advice.

“Never before in my 45-year career have I seen such a far-reaching public recommendation issued by any governmental agency without a single source of data or information to support it,” Osterholm said in response to the CDC’s lack of evidence on the effectiveness of cloth masks. “This is an extremely worrisome precedent of implementing policies not based on science-based data or why they were issued without such data.”

Osterholm criticized the “increasing number of poorly conducted and inadequately reviewed studies getting published in rapid succession.” Remarking on the evidence that we do have regarding cloth masks, Osterholm found it to be “limited and indirect.”

Dr. Lee Riley, division head of Infectious Diseases and Vaccinology at UC Berkeley, believes that a combination of the low rates of COVID-19 in Asian countries and experiments on the filtering ability of different materials prompted a gradual change in the scientific community’s attitude towards facial coverings.

He surmised that mask recommendations began as the CDC and public health officials observed how quickly the pandemic was controlled in Asian countries where wearing masks is a regular practice.

Citing a video from an applied chemistry professor in Japan who tested the efficacy of three types of “masks,” Riley described how even a covering made from three paper towels folded into six layers was successful in filtering out 80 percent of particles in the air—though there were no SARS-CoV-2 viruses in the air and this test was not a peer-reviewed experiment. In the video, Dr. Tomoaki Okuda of Keio University measured different materials’ ability to block airborne particles using a Scanning Mobility Particle Sizer.

Osterholm cautioned against such reasoning and experiments as stand-ins for hard evidence. In his podcast, he points to widespread testing, extensive contact tracing programs and quarantine measures in Asian countries as important measures that reduced the virus’s spread. He also addressed such lab experiments, noting the lack of evidence for smaller aerosolized particles the size of those potentially exhaled by the infected.

He believes masks could play a role in slowing the epidemic but wants to ask additional scientific questions regarding the virus’s spread so the public can make informed choices. One of the most important questions, he said, is how infectious the virus is in the air and how time and dose play a role in infection.

“My whole challenge has not been about whether you wear a mask or not,” Osterholm said in an interview on Monday. “I’ll throw the kitchen sink at this if it will help. The challenge is, ‘How well do they work?’ so that the public knows what level of protection they’re getting.”

Two of the epidemiologists interviewed for this article cited a new study, published on June 1, as important evidence regarding the effectiveness of masks in preventing person-to-person transmission.

Dr. Derek Chu at McMaster University in Canada, along with researchers from universities worldwide, conducted the research in the June study on behalf of the international research team, COVID-19 Systematic Urgent Review Group Effort. The study found that “face mask use could result in a large reduction in risk of infection.” 

Again, however, the study does not evaluate the cloth coverings that most people are using. It did, however, combine results of 172 studies from 16 countries and 6 continents to do a meta-analysis, which means it looked at the data to establish trends. The study found that certain types of institutional-grade masks, as well as physical distancing and eye protection, could reduce the risk of infection. Respirators and N95s were found to be most effective, but disposable surgical masks and similar products (such as reusable 12–16-layer cotton masks) also seemed to reduce risk.

Its findings supported “physical distancing of 1 m or more” and advised that “Optimum use of face masks, respirators, and eye protection in public and health-care settings should be informed by these findings and contextual factors.” It also cautioned that more study is needed: “Robust randomized trials are needed to better inform the evidence for these interventions, but this systematic appraisal of currently best available evidence might inform interim guidance.”

On June 5, just days after the study was published, WHO updated its guidelines on masks recommending for the first time that face coverings be worn in public when physical distancing is difficult. Those over the age of 60, it now advises, should wear medical masks, while others should wear non-medical masks.

The recommendation for non-medical masks came with a caveat: there is not any “high quality or direct scientific evidence” to support their use, but the recommendation noted that research on asymptomatic transmission, observational evidence regarding mask use by the general public, individual values and the difficulty of physical distancing led WHO to update its guidelines. It’s unclear how WHO’s Van Kerkhove’s new revelations on asymptomatic transmission will impact these guidelines. 

For now, its guidance noted that non-medical masks “should only be considered for source control (used by infected persons) in community settings and not for prevention.”

In contrast to the CDC’s recommendation for cloth masks, WHO recommends a three-layered mask composed of an inner absorbent layer (such as cotton) to retain droplets, an outer layer of hydrophobic material (polypropylene, polyester or their blends) to filter and a middle layer made with cotton or hydrophobic material.

WHO also warns that promoting mask use could result in a “false sense of security, leading to potentially lower adherence to other critical preventive measures such as physical distancing and hand hygiene” and cites the potential for increased self-contamination as people manipulate the masks. Epidemiologists continue to advise that masks be worn in conjunction with other practices, such as physical distancing and regular hand-washing. 

Despite CDC and WHO recommendations, there remain plenty of state public health officials who have yet to require masks.

Facial coverings are currently mandated in some capacity in all but two Bay Area counties, though Solano and Santa Clara counties both strongly recommend their use. The term “face covering” is often loosely defined and includes everything from medical-grade masks to bandanas. 

At the national level, 25 states have some kind of facial covering requirement, not including local guidance, according to CNET. These requirements vary in severity. Many pertain to businesses and employees doing essential work, but some apply to individuals in public.

The efficacy of masks outdoors is also unclear.

As the weather warms up, beaches, parks and other outdoor spaces are becoming increasingly populated. Protests have also filled the streets with demonstrators. Epidemiologists believe the likelihood of infection is reduced outdoors as wind dilutes virus droplets in the air. 

Though being outdoors may decrease risk, many epidemiologists say individuals should still be wary of person-to-person spread when in close contact with others. Moreover, they warn, face coverings, especially cloth coverings, are not a substitute for other critical measures such as physical distancing and good sanitation.

For those confused by changing guidelines, Dr. Arthur Reingold, department head of epidemiology and biostatistics at UC Berkeley, had one message: “For a completely new infectious agent, like the novel coronavirus that causes COVID-19, the reality is that there’s an incredibly fast evolution of knowledge. Virtually every day, new information becomes available that might change our understanding of things. It would be foolish if we didn’t change our recommendations as we acquire more knowledge . . . I personally do put faith in what comes from the CDC. There are a lot of really smart, hardworking, well-intentioned people there doing their best to inform the public about how to stay safe.”

The challenge, Osterholm said, is that “we owe the public a lot more information than either ‘everybody masks’ or they don’t.”

We sort through the studies and talk to the experts so that you don’t have to. If you haven’t already, support our reporting today.