It emerged last night that Boris Johnson ignored advice from the government’s committee of scientific advisers, the SAGE group, to institute a package of national measures, including a “circuit-breaker” lockdown, closure of hospitality and any mixing between households.

The various measures in documents released by SAGE are justified supposedly by their impact on the national R number. A circuit-breaker was credited with “reducing Rt to below 1.” As the virologist Professor Hugh Pennington told me recently, a national “R number” has “the ring of science” and not much else. SARS-Cov-2 is cluster-driven – it exhibits exponential growth only in limited circumstances. It is also an “overdispersed” virus – one case does not necessarily lead to another.