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.