Assessment of the impact of the use of face coverings on
infections, hospital admissions and deaths.
Using this stepped approach, we assessed the potential impact of face
coverings on (a) number of current and consequent future infections, (b)
number of hospital admissions and (c) number of hospital deaths.
The ONS Coronavirus (COVID-19) Infection Survey pilot11 reported the modelled daily incidence infection
rate for each week based on exploratory modelling. At the time of
writing, the modelling used to calculate the incidence rate was a
Bayesian model and used all swab test results to estimate the incidence
rate of new infections for each different type of respondent who tested
negative when they first joined the study. This can be multiplied by 7
to give an expected total number of new community infections each week
from all sources. The number reported in the week before the imposition
of face coverings on the 24th July 2020 was taken as
the baseline for this study
NHS England 27 reported daily hospital COVID-19
admission data which included all people admitted to hospital who
already had a confirmed COVID-19 status at the point of admission and
those who tested positive in hospital after admission. Inpatients
diagnosed with COVID-19 after admission were reported as being admitted
on the day before their diagnosis. Admissions included data from all NHS
acute hospitals and mental health and learning disability trusts, as
well as independent service providers commissioned by the NHS. It was
assumed that patients would be admitted 7 days after their original
infection and so a ratio of hospital admission to the previous week’s
number of infections enabled us to calculate an infections admission
rate (IAR). However, in these admitted patients, infections might have
occurred within either the community, care homes or hospital so we
conservatively assumed that 50% of this infection hospitalisation rate
occurred within the community.
NHS England, 28also reported daily the deaths of
patients who had died in hospitals and had either tested positive for
COVID-19 or where COVID-19 was mentioned on the death certificate. All
deaths were recorded against the date of death. In our analysis, the
length of stay in hospital before death was assumed to be 2 weeks so the
ratio of total deaths to the total admissions 2 weeks previously give an
estimate of hospital admissions fatality rate (AFR). We conservatively
assume that the AFR from community admissions are similar to those from
care homes and hospital infections.
The benefit of any mitigation measure was assessed not only as those
avoided directly but also those consequent future infections. We
estimated this based on the re-infection rate (R-value) and re-infection
cycle time, over a defined period (three months). We utilised three
months as, by the end of this period, the situational outlook would
likely be reviewed. European Centre for Disease Prevention and Control24 report viral RNA shedding peaking in the second
week after infection so a conservative re-infection cycle time of 8 days
was applied from 24th July 2020. At this time, the UK
Government reported an R-value range for the UK of 0.7-0.9 and a growth
rate was given as -4% to -1% as of 24th July
2020.25 Consequently, three R values; namely 0.8 (the
accepted level at the time of the introduction of mandatory face
coverings), 1.0 (a worsening to equilibrium) and 1.2 (the pandemic
restarting) were used in our analysis. For each of these, we calculated
the total number of consequent future infections that could be expected
to flow from the original infections.
Baseline effectiveness of face coverings and the IRS calculated above
for retail outlets and public transport was applied to each scenario to
calculate the expected infections, hospitalisations and deaths over the
next 3 months. The sensitivity of the results to the assumptions on
face-covering effectiveness was tested by calculation the above for no
face coverings (0%), 20%, 40%, 60% and 80%.