Step 3: Impact of the use of face coverings.
The effectiveness of face coverings in reducing infections will be dependent on two broad factors: (i) the proportion of infections that are due to aerosols and other airborne routes of transmission and, (ii) the efficacy of face coverings of reducing the spread of such airborne-associated infections. Neither of these is likely to be 100%.
Face coverings are unlikely to be effective in mitigating against all transmission routes. The World Health Organisation (WHO) published a detailed assessment of routes of transmission.24 The European Centre for Disease Prevention and Control states that infection is understood to be mainly transmitted via large respiratory droplets.25 However, the proportion of infections caused by airborne or other routes that could be prevented by face coverings, while less than 100%, is difficult to quantify. Indeed, 80% might be considered a conservative estimate.
Furthermore, the efficiency of face coverings in regard to preventing airborne transmission is likely to be highly variable,26 not least due to the wide range of types of face coverings used (from scarves to surgical-grade masks), and their correct usage (as emphasised in UK government guidance1). Indeed, laboratory-based experimental data from van der Sande et al suggests that home-made face coverings offered around 29-78% protection against aerosol transmission over short periods, while surgical masks provided 50-91% protection.26 Efficiency in population settings, and in cases of prolonged contact, is likely to be lower and more variable than these estimates. However, on the other hand, if two people who come into close contact are both wearing face coverings, infection risk is likely to be further reduced.
Combined, the reduction in infection risk associated with the use of face coverings were modelled as using a range of values covering estimates (20%, 40%, 60% and 80%) as example scenarios.