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Covid-19 Masks transmission

Face masks 2: How effective are cloth face masks?

Face masks covering the nose and mouth are designed to act as a barrier to reduce viral spread. If people who are infected wear a face mask, is that enough to stop them infecting other people? In this post I review the evidence that the wearing of ‘normal’ cloth face masks is a good enough barrier to prevent passing infection to others.

To recap from my post last week:

In my first post on face masks last week, I posed the key question: “How much can the widespread  wearing of face masks reduce the spread of infection in the population?”  I argued that even if everyone wore face masks, the number of new cases in any area  would also depend on several other factors:

  1. The effectiveness of the chosen masks as a barrier to passing on the virus
  2. The safe and appropriate use of any face masks worn 
  3. The level of adherence to other mitigation measures such as social distancing, hand washing etc
  4. The background risk of infection in a local population. 

In this post I address the first question, on the success or otherwise of face masks as a barrier to the passage of virus

Who is protecting whom?

Source: Mario Tama/Getty
  • The research on the effectiveness of face masks as a barrier is based on studying how they stop the virus spreading from an infected person. 
  • It is very difficult (ethically and logistically) to do a study  to show that wearing a face mask is an effective barrier to receiving the virus from an infected person
  • It is a reasonable assumption that the barrier is equally effective in both directions  – although the receiver can also get infected via the eyes or passing droplets by hand to their nose or mouth

Surgical masks

  • Much of the research is on the effectiveness of surgical masks and their use in health care settings
  • Surgical masks have non-woven fabrics (no holes) and are disposable 
  • Several studies show that surgical masks are very effective at reducing the risk of infection
  • As an example, research  from Hong Kong published in May studied people all of whom had a positive swab test either for Covid-19 (or  for common cold or flu).
    • They then studied the proportion of those people who were breathing out viruses, both without and with a face mask.
    • They measured if there was shedding of the virus in both large droplets and small droplets (aerosols*)
    • Of those with CoVid (blue bars in graph below), who did not wear a face mask,  30%  shedded virus as droplets and 40% as aerosols.
    • With a face mask none of those with Covid-19 shed the virus in either form
    • Interestingly in this study, surgical masks were more effective as a barrier to Covid-19 than to the common cold or flu viruses

What about cloth masks?

  • Disposable surgical masks are not a practical proposition for use in the general  population
    • The number of masks needed would be enormous
    • The cost for individuals or organisations is excessive
    • They are not comfortable for wearing for long periods such as a school or working day
  • In SARS and in other pandemics, authorities had suggested that cloth masks are the only feasible alternative for the general population who are not at the same risk as say health care workers
  • Several previous research studies, from SARS 1 and other epidemics, have shown that surgical masks are far superior to cloth masks, but might cloth masks be good enough?

Challenges in assessing the value of cloth masks

  • For sure it is more difficult to block aerosol spread than droplet spread in woven cloth materials even if we can’t see the holes
  • Holding up a mask to the light and seeing if nothing shines through is helpful but this is not proof it would filter out the smallest droplets
  • It is also clear that the likelihood of transmission of virus even through a well-fitting cloth mask is increased by:
    • the amount of virus
    • how much speaking/shouting/coughing etc takes place
  • Thus, it is really difficult from the available research to be definitive about how effective cloth masks are as a barrier.

There are cloth masks and cloth masks!

  • There are several designs of cloth masks which make it difficult to draw conclusions about cloth masks in general 
  • Factors that vary include:
  • Number of layers
  • Shape: conical (see picture) or traditional folded
  • Whether there is an additional filtration layer
  • In one recent review comparing 15 different designs and materials of cloth masks, the effective filtration rate varied from 28-90%, though 80% is frequently achieved

What is my summary  about cloth masks?

  • No cloth mask stops 100% of droplets 
  • There are inevitable uncertainties in the precise protection afforded by any individual mask 
  • Depending on the construction of the mask, an 80% reduction might be achieved but that might vary with the size of the droplets
  • The more an infected person shouts, coughs etc then the harder it is for a cloth mask to be effective
  • The longer the mask is worn the less effective it might be – but regular washing helps
  • The effectiveness of masks obviously depends on how they are worn, and whether they let out droplets round the side

Conclusion

  • Cloth masks are not the magic bullet to stop an infected person spreading the person, or protect an uninfected wearer from being at risk from others.  
  • Widespread usage might still have a major role in lowering the overall rate of infection in the population.  The size of this effect will be explored in my next post

Interested in further reading:

  1. https://www.medrxiv.org/content/10.1101/2020.04.17.20069567v4 How the effectiveness of different cloth masks can be measured.
  2. https://www.nature.com/articles/s41591-020-0843-2#Sec3 Study of the effectiveness of surgical mask against Covid-19. 
  3. https://files.fast.ai/papers/masks_lit_review.pdf A comprehensive  review of all the studies on the effectiveness of face masks.

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