Covid-19 Masks

Face masks: How much can they reduce the spread and duration of the epidemic?

There has been a continuing debate about the benefits from using face masks since the start of the epidemic.  The World Health Organisation was initially unconvinced of their benefit but recently changed its mind.  Socially and politically it has become the number one issue.  

Can enhanced wearing of face masks succeed where other means of managing the epidemic including  lockdowns, social distancing and track and trace have not?

This key question is surprisingly difficult to answer  but in this post I review what evidence is needed. 

Isn’t questioning  the benefits of face masks a no-brainer?

  • Covid-19 spreads from  an infected person by droplets,  with concentrations of virus particles increasing from normal breathing to shouting, sneezing  and coughing
  • Covid-19 spreads to a non-infected person either directly into their nose/mouth/eyes or indirectly by droplets picked up on the hands
  • Most spread is from people who are asymptomatic at the time they can pass on the infection.
  • Therefore people can reasonably say:
    • “If everyone around me wears a face mask, I am protected”
    • “If I wear a face mask, I am at least partly protected from others who may be infected” 

What’s the problem?

  • Not all face masks are effective at preventing spread
  • The way face masks are used may make them ineffective and possibly increase the risk of both causing and catching infection


  • Wearing a face mask for a long period of time is uncomfortable
  • As a society we rely for social interaction on seeing people’s faces – consider the issues raised by religious practice in this regard

And in addition:

  • Behavioural psychologists are concerned that by wearing face-masks, there is a relaxation of other protective behaviours such as social distancing.
  • It is reasonable to ask: “what is the additional value of wearing face masks if people adopt all other protective behaviours (distancing, hand washing etc)?”

The challenge from a research perspective in quantifying the benefits from face masks therefore comes from variability in:

  • Mask effectiveness
  • Safe mask use
  • Adherence to other protective behaviours
  • Background rate of infection in the population

Widespread wearing of face masks theoretically should  reduce risk to individuals, but in the real world it may not reduce the rate of infection in the population.

Is there a parallel with the vaccine question?

  • The current vaccines work and produce antibodies
  • But before being used we are having to await the results of experimental studies (clinical trials) where two otherwise equivalent groups of people are randomised either to active or inactive vaccine  to see over time how many become infected.  
  • We cannot do such a randomised study of face masks easily and thus have to rely on indirect evidence

The epidemiological train of thought!

  • Do current face masks do “what they say on the tin”, ie prevent transmission of viral particles:
    • This is not a yes/no question but the issue is more by how much is transmission reduced
    • How does this vary with type of mask?

There is no shortage of experimental data, eg direct measurement of the concentration of virus in air breathed or coughed out, to answer these questions

  • Assuming that these data confirm that most face masks in common use are a ‘sufficient’ barrier, there are two types of indirect information that can be looked at:
    • Ecological studies. These are studies comparing rates of infection between whole countries with different mask wearing behaviours.  Care is needed as a high mask wearing, low infection risk country might differ from other countries in other ways apart from mask use
    • Observational studies. The rates of infection are analysed in individuals who are believed to have similar exposures to the virus who are asked about their use of masks.  
  • Such studies may not capture the full range of current and future behaviours. These can be estimated from:
    • Modelling studies. As mentioned above there are so many factors that vary which will influence the impact of mask wearing on the rate of infection.  Different plausible scenarios can be compared by adjusting the level of, for example, of mask wearing and adherence to social distancing to estimate their combined  impact on the infection rate.  These simulations have emerged with some interesting insights on the relative value of masks


  • The data on the research referred to above will be summarised by me over the next week
  • I hope readers will not feel let down having read so far and not getting the full story in this post!
  • A complex question deserves not giving a simplistic answer
  • Spoiler alert – these data are reasonably  supportive of the additional benefit of mask wearing. 

3 replies on “Face masks: How much can they reduce the spread and duration of the epidemic?”

Trish is a colleague of mine in Oxford and has published an excellent summary of the data on masks which I will be referring to in my next post. She advocated from the start on this and even advised us all how to make masks from old tee shirts


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