COVID 19 – Masks on or off?

April 17, 2020

Tom Jefferson, Carl Heneghan

Masks act as a symbol for society – you are protected. The evidence says you may not be.

Seventeenth-century depictions of “plague physicians” show them wearing a long beak-like mask, tied with rope and carrying a disinfectant flame in the beak.

The flame was supposed to prevent plague miasmata from entering the physician’s body, but carry a mask he did.

The question which is now being hotly debated is whether in the current pandemic we should all be wearing wear masks or not and if so in what circumstances.  

Thirteen years ago one of us co-authored what was (to our knowledge) the first systematic review of physical interventions to prevent the spread of respiratory viruses. The review was updated in 2009 and again in 2011 and is now accessible free of charge from the Cochrane website.

Our first review had an evidence nucleus of 7 case-control studies carried out in the Far East during the 2003 SARs 1 epidemic. The studies were mostly carried out in an emergency situation with researchers under a lot of pressure to provide answers for decision-makers.

The evidence they provided showed that multiple interventions such as barriers, distancing and hygiene decreased the risk of infection. However, the bulk of the evidence came from healthcare workers settings, with only one case-control looking at family clusters, within their dwellings.  In the following updates, we added a few more observational studies and the number of randomised studies increased, but not by much.

This year we have produced a further update. Because of the expected greater number of studies and the haste of our funders for answers, we decided to split the review update into two parts. We were confident that we would have a number of trials to answer our questions with greater precision than case-control studies. The first part includes evidence of the effects of face masks, eye protection and person distancing as single interventions, not in combination.

Evidence from 14 trials on the use of masks vs. no masks was disappointing: it showed no effect in either healthcare workers or in community settings. We could also find no evidence of a difference between the N95 and other types of masks but the trials comparing the two had not been carried in aerosol-generating procedures. 

However, our findings cannot be the final word. 

For starters, most of the trials were poorly reported and carried out during seasons of influenza-like illness when viral circulation is variable, but probably way below that in the Lombardy areas at the beginning of March. The design and execution of some of the trials were also questionable and as most were cluster-allocated, blinding was difficult, if not impossible.

The trials carried out “in the community” were in fact in specific settings such as halls of residence, family clusters or worshipping pilgrims.

The evidence was also mostly generated by two groups of researchers and the harms of masks were under-reported and no one reported in detail on non-compliance and possible reasons for it.

It is often more difficult to breathe while wearing masks (particularly the respirator masks), which can exacerbate other health issues. An overview of 84 articles found that protective facemasks also negatively impact respiratory and dermal mechanisms of human thermoregulation, making it hard for many to wear constantly. 

Thinking you’re protected, means you may put yourself at higher risk, and as individuals, we will change our behaviour in response to the perceived levels of risk. We are more careful if the level of risk is high and less careful if it is low. Measures we can take can include washing hands, avoiding touching, social distancing, school closures and self -isolating when unwell. You may also end up touching your face more often. 

A mask can become dirty with excessive moisture, and contaminated with airborne pathogens. And because your voice is muffled; individuals may have to get closer to people, particularly the elderly, to hear from you.

If they are recommended, panic might ensue if mask availability is limited; we can barely get sufficient supplies for our healthcare workers. Cloth masks, where there is no evidence of effectiveness, might be seen as inferior to respirator masks. Further, public purchasing of masks might limit the availability in health care settings. Not everyone can afford them — they will need to be publicly funded, on a regular and ongoing basis. 

Taking into account the observational evidence from the previous coronaviridae outbreaks we should certainly use all the precautions for exposed people, especially our healthcare workers.

But what of the folk walking down the road, going to the supermarket or watching the ducks in the pond? 

The answer is simple: we do not know. That being the case, and since there is a pandemic underway we are in an ideal situation to carefully record “natural experiments” on a global scale, comparing rates of infection and transmission between states at different stages of lockdown and with different masking and distancing policies.

Ideally, we should carry out global trials testing the effects of the absence of masks, but we doubt politicians would be willing to take the risk that Max Von Pettenkopfer did when he swallowed a suspension of Vibrio cholerae to test the bacterial causal theory.        

So we got into this situation unprepared with a faulty evidence base and hotly debated practices, after two decades of “pandemic preparedness”.  

Society has choices: find out if they work or not, and in what circumstances, or recommend their use, with or without other measures, or use those non-pharmacological interventions where there is more evidence of benefit.


Carl Heneghan is Professor of Evidence-Based Medicine, Director of the Centre for Evidence-Based Medicine and Director of Studies for the Evidence-Based Health Care Programme. (Full bio and disclosure statement here)

Tom Jefferson is an Epidemiologist. Disclosure statement is here