Face masks in the covid-19 crisis: caveats, limits, and priorities

We agree with Greenhalgh and colleagues that decision making should be based on scientific knowledge, but when knowledge is incomplete, as in the current pandemic, judgments based on precaution and pragmatism become increasingly necessary.1 The Association of Schools of Public Health in the European Region’s report launched on 24 April recommends the use of face masks by the general public, with clear caveats, limits, and priorities.2

Non-medical grade “social” masks must be one element alongside other non-pharmaceutical measures (personal hygiene, physical distancing, and so on). If the aim is to recommend or require mask use for the entire population, social masks will have to be used.3 If not, there will be an aggravated shortage of professional masks for health and other critical service workers and others at high risk.

The need and function of masks differ at various phases of the epidemic. The widespread use of masks has not been verified before community transmission or at the end of sustained transmission. In the epidemic growth phase, it might mitigate viral transmission by asymptomatic patients and thus limit the epidemic’s growth rate. But isolation and physical distancing are most important to control transmission at this point, and social mixing with masks should be discouraged. Masks should be used from the consolidated start of the decline in contagion as part of a phased reduction of lockdown measures and a return to economic activity. If widespread use of masks is implemented, there must be a strong public training campaign by health authorities with quality assurance processes.

More research is needed on the material properties of non-medical, social masks. We have been asking the wrong questions.4 They are not for the protection of the mask user, as tested in clinical trials. They are for the protection of the wider public. Given the documented high levels of asymptomatic carriage5 and the prolonged period of virus excretion before and after illness, the risk of inadvertent spread is very high and can only be mitigated by use of social masks alongside frequent handwashing, physical distancing, and other social measures.