Step 2: Infection risk by activity.
We calculated an Infection Risk Score (IRS) for a number of common activities. Firstly, based on location, we categorised daily activities into the following: home, work, public transport, retail outlets, other activities (indoors) and, other activities (outside). We calculated the average length of time spent per day on each of these activities. This was based on the United Kingdom Time Use Survey, 2014-2015,18 as quoted in a Scottish government report,19 and a Resolution Foundation report in July 2020.20 This describes average minutes per day spent by those aged 16 years and over on the following activities: (a) Paid work, (b) Unpaid work (sub-divided into housework; shopping, services and household management; childcare; travel; construction and repairs; and voluntary work), (c) Study and (d) Leisure: (sub-divided into TV and other leisure; social life, culture and entertainment; and sports and outdoor leisure). Each of these categories was assigned to one of the groups listed in Table 1, with 8 hours allocated to sleep (based on the Resolution Foundation report,20 which quotes the United Kingdom Time Use Survey as assigning 8.5 hours to sleep for the 18-64 age group). For the modelling, we assumed that 50% of all travel time was using public transport and that the category defined in the United Kingdom Time Use Survey as ‘shopping, services and household management’ comprised 50% of time allocated to various forms of shopping, including for groceries, clothing and that undertaken for leisure. We realise that these are likely to be overestimated but elected to take a conservative approach.
Each activity was then assigned a risk of infection. This was based on a risk stratification approach used by the Texas Medical Association,21 which was then sense-checked using ONS data which allows assessment of the infection risk associated with working from home versus working in other environments.2223
These two components were combined to calculate the activity IRS, this was then summed. The % of this total allowed us to assess the percentage contributions to the risk associated with each activity, all other aspects assumed being equal..
We elected to use conservative over-estimates of the IRS associated with transport/retail activities. It should be noted that having to wear face coverings may inhibit frivolous or spontaneous travel and shopping activities, and hence the proportion of time spent on these activities following the implementation of the mandatory policy may decrease, at least after an initial surge following the easing of restrictions.