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.