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.