Importance of untested infectious individuals for the suppression of COVID-19 epidemics

Francisco Perez-Reche; Norval Strachan

In order to study the effect of isolating tested and untested individuals, we extended the model shown in Figure 1 to include compartments for isolated tested and untested infected individuals (see Model 2 in Methods). Interventions are parametrised by the fraction of tested (untested) infected individuals that are randomly selected to be isolated, ( ), and the rate at which they are isolated after testing/reporting. Isolation strategies leading to eradication of infection satisfy the condition ℛ 0 < 1, where ℛ 0 is the reproduction number for Model 2 (see Eq. (6) in Methods). Given the significant reporting delays we found, isolation has to be fast after testing positive. In particular, we found that isolation after an average time of −1 = 1 day could only lead to eradication in Germany; for other countries, ℛ 0 remains larger than 1 for any and . Following this, in Figure 6 we show results for a scenario in which individuals are isolated after an average time of −1 = 1/2 days. In this case, eradication is possible in all the studied countries if both and are large enough, i.e. if enough tested and untested cases are isolated. The estimated boundaries separating the eradication region (ℛ 0 < 1) from the epidemic region (ℛ 0 > 1) are different for different countries but differences are statistically less marked if confidence intervals are taken into account.