Aim: To use theory and practice to show how disease progression and regression can be described pre- and post-lockdown using an attack–sustain–decline–respite (ASDR) model and investigate how pre-lockdown disease prevalence and household size impacts on the effectiveness of lockdown . Materials and methods: Computed tomography (CT) scans from major trauma patients (considered as a random population sample) from the radiology department of St George ’ s University Hospitals NHS Trust, London, have been used to explore COVID-19 disease at the population level .

Results: At lockdown on 23 March 2020 in the catchment area of St George ’ s University Hospitals NHS Trust, an earlier paper showed that there was a high prevalence of disease of> 20% . With further follow-up and at the end of lockdown, it have been now estimated that around 57% of the population had been affected, which was similar to that predicted from a simple model based on average household size and prevalence at lockdown . With an average household size of around three persons, there was a 2-week sustain period and a 5-week decline period before the prevalence of the disease returned to background levels .

Conclusions: The present results suggest that the effect of lockdown is dependent on the disease prevalence at the start of lockdown and the average household size . It may therefore be important to lockdown early in an area with a high average household size . This paper is the second in a series of papers to show how radiology measurements of major trauma patients can be used to help monitor the spread of the COVID-19 pandemic.