Masks are going to be mandatory – we need clear messaging now to ensure it happens | Daniel Howdon

With good sense having belatedly prevailed, the policy's success is dependent on the degree and quality of compliance

On 24 July it becomes compulsory to wear face masks in shops and supermarkets in England. What determines how we can expect such measures to play out? Largely, three things: how confident we can be in the available evidence, the size of the estimated effect among a compliant population, and the degree of compliance.

While a lot of attention has focused - rightly - on the first two of these, the third is equally important. Compliance not only encompasses questions of whether there is adherence to the guidance, but also the quality of the adherence. It is instructive, then, to compare the case of face masks with that of another very recent public health intervention: the recommendation of handwashing to stop the spread of Covid-19.