Step 1: Source of infection.
We considered the impact on the number of infections within the
community rather than in hospitals or care homes, as these are where
people using retail and public transport will be most reflected. There
will be some cross infections but the level of this is beyond the scope
of this analysis but is likely to be small.
Given that it is unlikely that people displaying more severe symptoms of
infection would use public transport or visit retail outlets, we then
utilised ONS and wider literature data to estimate the proportion of
asymptomatic or pre-symptomatic cases.
The ONS data suggests that only around one-third of individuals testing
positive for COVID-19 on a swab test reported having
symptoms.14 This was based on self-reported symptoms
and therefore may be an underestimate. According to Diana et al,
transmission by pre-symptomatic people accounts for around 40-60% of
transmissions and asymptomatic cases accounts for around 15% of
transmissions, indicating that between 55 and 75% of infections may be
derived from people without symptoms.15 While posted
on the preprint service website, medRxiv, early in the pandemic, these
data were reviewed and assessed by the Centre for Evidence-Based
Medicine on 23rd July 2020.16According to Yin and Jin, there is no difference in transmissibility
between those with and without symptoms.17 For the
modelling, we used a conservative estimate of 80% of infections from
pre- or asymptomatic cases.