2020 will be remembered by contemporaries like 1989, 2001 and 2008 were. These were years of exogenous shocks. Societies and economies were roiled by big external events. In 1989 it was the collapse of the Soviet Union. In 2001, terrorism. In 2008, the sub-prime mortgage crisis. Each event triggered a fallout that cast a long shadow over the succeeding decade. All manner of dislocations, reconfigurations, adjustments and adaptations followed as societies scrambled to cope with a big jolt and make sense of it. In 2020 a similar exogenous shock was provided by the COVID-19 (SARS-CoV-2) virus.¹ It set off a cascading series of interventions, restrictions and disruptions that culminated in the lockdown of whole societies and economies. What happened, why did it happen and was it justified? To answer these questions, we must begin with the virus itself.

The degree of virulence of a virus can be measured. The standard measure—the R₀ [R-zero] or basic reproduction number of a virus—is the average number of persons that an average infected person can potentially infect. The effective reproduction number (R_E) tells us how many persons an average infected person actually infects at a given point (or points) in time.² The R₀ number assumes that there are otherwise no measures or conditions that limit the virus’ communicability. The profile of the susceptible population, the behaviour of transmitting agents, and degrees of immunity can all affect the rate of spread of a virus in practice. An R₀ below 1 means that an infected person infects on average less than one person—and when that happens the virus struggles to reproduce itself and spread.

The basic reproduction number varies depending on the virus. The R₀ of mumps is high (4–12) though not as high as measles (12–18) or chicken pox (10–12). The 2019 H1N1 influenza R₀ was 1.4. Seasonal influenza ranges from 0.9 to 2.1 (Eisenberg 2020, February 5). The early estimates of the R₀ of COVID-19 varied widely. Oxford University’s Centre for Evidence Based Medicine on April 14 2020 cited a median figure of 2.63 and an R₀ ranging from 0.4 to 4.6 (Aronson et al. 2020). This was based on nine early studies from Wuhan, Shenzhen and South Korea. In comparison, the 1918 pandemic flu had an R₀ of 2 to 3.

By assuming a given R₀ number, epidemiologists can derive from that a presumptive figure for the level of ...