There are several possible explanations for what was a significant error in policy and advice early in the pandemic. These include: • the lack of adequate data on the spread of covid-19, as a result of the inadequacy of the UK testing operation; • overreliance on specific mathematical models when there were too many uncertainties; • assumptions about public compliance with rules that turned out to have underestimated the willingness to conform even for long periods; • the composition of SAGE suffered from a lack of representation from outside the United Kingdom; and • a preference for a particular UK approach may have been favoured above advice based on emulation of what was being pursued elsewhere.