School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK:

Objective To replicate and analyse the information available to UK policymakers when the lockdown decision was taken in March 2020 in the United Kingdom.

Design Independent calculations using the CovidSim code, which implements Imperial College London’s individual based model, with data available in March 2020 applied to the coronavirus disease 2019 (covid-19) epidemic.

Setting Simulations considering the spread of covid-19 in Great Britain and Northern Ireland.

Population About 70 million simulated people matched as closely as possible to actual UK demographics, geography, and social behaviours.

Main outcome measures Replication of summary data on the covid-19 epidemic reported to the UK government Scientific Advisory Group for Emergencies (SAGE), and a detailed study of unpublished results, especially the effect of school closures.