Contact data and SARS-CoV-2: Retrospective analysis of the estimated impact of the first UK lockdown

To combat the spread of SARS-CoV-2 in March 2020 the United Kingdom (UK) announced a series of restrictions on social interaction, culminating with the introduction of lockdown measures. Estimation of lockdown effectiveness using pandemic models relied on the availability of contact data and choices on how to structure models accordingly. We revisit the Cambridge/Public Health England real-time model (RTM), which was routinely implemented during the pandemic to monitor its development and produce short-term projections. To derive contact matrices, Google Mobility weekly contact data and school attendance data from the Department for Education were combined with information from the POLYMOD study and the UK Time Use Survey. These matrices were combined with susceptibility and transmissibility parameters to estimate effective reproduction numbers, which were taken as indicators of transmission trends. We explore alternative formulations of the RTM, which make fuller use of the available contact data, and assess the impact of each formulation on the conclusions of lockdown effectiveness. Results show that the estimated impact of the lockdown remains unchanged, but also uncover previously uncaptured early epidemic dynamics. This highlights the importance of the timely availability of contact data in understanding transmission dynamics during the early stages of an epidemic and assessing the effectiveness of interventions.