The World Health Organization (WHO) is standing by its general recommendation that healthy members of the population need not wear face masks to protect against the spread of COVID-19. However, some experts are suggesting large-scale implementation of facial coverings may actually be useful in curbing viral transmission.

Over the last couple of months the general messaging from most health authorities has consistently stated that unless you are sick, or caring for someone who is sick, there is no need to wear a protective mask. There is no evidence the virus is being spread via aerosol airborne transmission, so the consensus has been that healthy people simply do not need to wear protective masks. Basic hygiene and social distancing are the best strategies for avoiding the disease.

Many parts of the world are suffering major shortages in personal protective equipment, subsequently affecting the safety of frontline health care workers who are the ones that truly need the masks. So underpinning one aspect of the general recommendation has been an attempt to avert panic buying and hoarding of these necessary medical supplies.

At a recent press briefing Michael Ryan, Chief Executive Director of the WHO Health Emergencies Programme, stressed the shortage of mask supplies is not the primary reason the organization recommends against the mask wearing by the healthy general public. Instead he suggests there is little evidence of benefit and some concerns over misuse.

“There is no specific evidence to suggest that the wearing of masks by the mass population has any potential benefit,” says Ryan. “In fact, there’s some evidence to suggest the opposite in the misuse of wearing a mask properly or fitting it properly.”

In terms of personal protection, the WHO’s latest scientific briefing on COVID-19 transmission affirms there would be little protective benefit in healthy people wearing face masks. The brief notes that the most current evidence confirms the virus is most likely transmitted through respiratory droplets and direct contact with infected people.

There is no specific evidence to suggest that the wearing of masks by the mass population has any potential benefit Michael Ryan, Chief Executive Director of the WHO Health Emergencies Programme

Airborne transmission of the virus may be possible, but it mostly would occur in highly specific scenarios where the virus is shed in aerosol form and subsequently remains in the air for long periods of time. Droplets from a cough, for example, are larger particles that do not hover in the air but quickly land on surfaces in the immediate environment.

“In the context of COVID-19,” the WHO writes in the briefing, “airborne transmission may be possible in specific circumstances and settings in which procedures or support treatments that generate aerosols are performed; i.e., endotracheal intubation, bronchoscopy, open suctioning, administration of nebulized treatment, manual ventilation before intubation, turning the patient to the prone position, disconnecting the patient from the ventilator, non-invasive positive-pressure ventilation, tracheostomy, and cardiopulmonary resuscitation.”

It is this particular reason why supplies of personal protective equipment, specifically N95 protective masks, should be reserved for medical personnel.

Because there seems to be little chance of airborne aerosol transmission of COVID-19 the general recommendations remain, asking only sick people wear protective masks. However, this strategy depends on being able to effectively, and quickly, identify those infected individuals.

“If I knew who was asymptomatic and presymptomatic [for COVID-19], I’d triage the face masks to those individuals,” says epidemiologist Arnold Monto to ScienceMag. “We don’t know who these are.”

And it is exactly this gap in the knowledge leading some experts to advocate for population-wide mask wearing. In a recent interview George Gao, the director-general of the Chinese Center for Disease Control and Prevention, cited a lack of mass mask wearing as the biggest mistake the rest of the world seems to be making right now in regards to controlling the spread of COVID-19.

“The big mistake in the U.S. and Europe, in my opinion, is that people aren’t wearing masks,” Gao said in the rare interview. “This virus is transmitted by droplets and close contact. Droplets play a very important role—you’ve got to wear a mask, because when you speak, there are always droplets coming out of your mouth. Many people have asymptomatic or presymptomatic infections. If they are wearing face masks, it can prevent droplets that carry the virus from escaping and infecting others.”

The WHO may be correct in suggesting face masks do not necessarily protect healthy people from contracting the virus. But recommending only those infected with the virus wear masks implies we are currently detecting all of those people who contract COVID-19. A growing body of evidence is beginning to uncover large volumes of asymptomatic cases, and it is these cases that may contribute to broad spread of the virus.

The big mistake in the U.S. and Europe, in my opinion, is that people aren’t wearing masks George Gao, the director-general of the Chinese Center for Disease Control

A major study published in mid-March in the journal Science found a startling volume of undocumented cases were responsible for the initial epidemic spike in China in January. Using a computer model to simulate the spatiotemporal spread of the virus based on reported cases from 375 Chinese cities, the study found confirmed cases could not explain the viral spread. In fact, the study concluded 86 percent of infections must have remained undetected to justify the broad spread of the disease in January.

“The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected,” said co-author on the study Jeffrey Shaman, from Columbia University Mailman School. “Depending on their contagiousness and numbers, undetected cases can expose a far greater portion of the population to virus than would otherwise occur. We find for COVID-19 in China these undetected infected individuals are numerous and contagious. These stealth transmissions will continue to present a major challenge to the containment of this outbreak going forward.”

Several smaller, more localized recent studies have started to confirm suspicions mild or asymptomatic cases could be more prominent than previously suspected. A CDC study into an outbreak at an aged care facility in Washington tested almost every resident after a health care worker was confirmed with COVID-19. Out of the 30 percent of residents that tested positive for the virus, about half were either pre-symptomatic or asymptomatic on the day of testing.

The few places in the world conducting large-scale COVID-19 testing are uncovering small, but relevant, numbers of asymptomatic cases. Mass testing in both Iceland and parts of Italy have revealed small proportions of positive cases displaying no symptoms.

Tom Inglesby, director of the Johns Hopkins Center for Health Security, has called for broad non-medical mask use among the general population as a way to lower asymptomatic virus transmission. Inglesby notes widespread face mask use is not about individual protection but instead about slowing community spread from those unaware they are infectious.

“… given growing evidence that asymptomatic transmission is contributing to COVID spread, it's now time for CDC to make recommendation for public, non-medical mask use to try to diminish asymptomatic people from spreading the virus,” he suggests in a long Twitter thread. “It's critical that all medical masks (N95, surgical, procedural) be given to health care workers and emergency personnel – there is a terrible shortage of these masks, and so all of them need to go to those caring for COVID patients. But members of the general public should wear non-medical fabric face masks when going out in public in one additional societal effort to slow the spread of the virus down.”

Inglesby also notes countries with strong pre-existing face mask behaviors have so far proved best at swiftly containing spread of the virus. South Korea, Hong Kong, Japan, China and Taiwan all have strong face mask cultures, and while this certainly is not the sole reason these countries may be successful in managing the pandemic, it could be a factor.

A recent Boston Globe editorial from two Yale University researchers, Shan Soe-Lin and Robert Hecht, succinctly summed up the reasons why they believe all people should be wearing some kind of face covering when outside. Soe-Lin and Hecht are clear in reminding people that non-medical masks will not protect someone from infection if directly coughed on by an infected person, and they certainly do not replace behaviors such as hand-washing or social distancing, however, masks can stop a person from incessantly and unconsciously touching one’s face.

“Cover your face with a mask,” the pair suggest. “This will deny you access to your own face and make you conscious of how often you are tempted to touch your nose and mouth. A nonmedical mask will not protect you from a direct cough or sneeze from an infected person, but if you’re practicing good social distancing, any type of face covering is great protection from your biggest threat: your own hands.”