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%.