Figure 1: Model structure and transitions through the model compartments. Susceptible individuals (S) become exposed by effective contact with any infectious (red compartments). Exposed individuals progress through three stages, E1, E2, and E3, before illness onset. By testing and clinical diagnosis, both pre-symptomatic (E3) and unknown (U) infectious individuals can be detected and thereby enter compartment I. Severe cases requiring hospitalization (H) or intensive care (C) are included. Infectious individuals are removed from the chain of transmission when they recover (R) or die (D) from the disease. Compartments covered by the green shadow are detected and reported in data. Unknown cases (U) lead to unknown recoveries.
All the scenarios described below assume that (i) high incidence numbers lead to automatic contact reduction by making individuals on average more careful, (ii) effective contact rates are higher in winter than in spring/summer due to more frequent indoor activities [6], (iii) high prevalence leads to higher under-reporting due to limited test and contact tracing capabilities, and that (iv) immunity acquired by having contracted and recovered from the disease does not wane as evidence on loss of immunity is yet debated [7,8].
In what follows possible scenarios for reducing infectious contacts are considered. It should be emphasized that none of the scenarios simulates a shutdown similar to the one in spring 2020. The term “shutdown” is used here for lack of a better word to concisely describe the measures taken. Moreover, the term severity of restrictions can be more properly interpreted as amount of reduction in effective contacts .
Instead of planned wave breaker interventions, fixed for established periods of the year, one might also think of applying and relaxing intervention measures based on the reported case incidence. We have tested (simulations not shown here) also such an intervention strategy with triggered measures, assuming e.g. that an incidence of 50 cases per 7 days and 100,000 inhabitants triggers severe restrictions leading to strongly reduced contact rates, whereas lifting of these restrictions is triggered by the incidence dropping to 8 cases per 7 days and 100,000 inhabitants. One obvious assumption to include in the model is that control measures cannot be put in place instantaneously, but require a few days to be effectively introduced or relaxed. This leads to dynamics similar to that in scenario 2, but with shutdown periods not occurring at predefined intervals.