Trajectory of COVID-19 epidemic in Europe

Summary 1

The classic Susceptible-Infected-Recovered model formulated by Kermack and
McKendrick [1] assumes that all individuals in the population are equally susceptible to 3 infection. From fitting such a model to the trajectory of mortality from COVID-19 in 11 4

European countries up to 4 May 2020 Flaxman et al. concluded that “major non-pharmaceutical interventions – and lockdowns in particular – have had a large
effect on reducing transmission” [2]. We show that relaxing the assumption of
homogeneity to allow for individual variation in susceptibility or connectivity gives a model that has better fit to the data and more accurate 14-day forward prediction of mortality. Allowing for heterogeneity reduces the estimate of “counterfactual” deaths
that would have occurred if there had been no interventions from 3.2 million to 262,000, 11

implying that most of the slowing and reversal of COVID-19 mortality is explained by
the build-up of herd immunity. The estimate of the herd immunity threshold depends
on the value specified for the infection fatality ratio (IFR): a value of 0.3% for the IFR 14

gives 15% for the average herd immunity threshold.