When is lockdown justified?

How could the initial, drastic decisions to implement “lockdowns” to control the spread of Covid-19 infections be justifiable, when they were made on the basis of such uncertain evidence? We defend the imposition of lockdowns in some countries by, first, looking at the evidence that undergirded the decision (focusing particularly on the decision-making process in the United Kingdom); second, arguing that this provided sufficient grounds to restrict liberty, given the circumstances; and third, defending the use of poorly empirically constrained epidemiological models as tools that can legitimately guide public policy.