Projecting the Spread of COVID19 for Germany

Jean Roch Donsimoni; Rene Glawion; Bodo Plachter; Klaus Waelde

Starting with the transition from being healthy to sick, each individual in state 1 has a certain average number of social contacts per day. We capture this by an exogenous arrival or contact rate a. Once a contact takes place, there is a certain probability that the contact is with an individual that can infect the healthy individual. If we assumed random contacts with healthy and sick individuals, this probability would be (N 2 (t) + N 4 (t)) = (N 1 (t) + N 2 (t) + N 4 (t)) ; where N s (t) is the (expected) number of individuals in state s at t: This probability assumes that all N 2 (t) individuals that are sick can infect healthy individuals and a constant and exogenous share of healthy individuals in state 4 can also infect healthy individuals from state 1: This parameter captures the idea of "quiet infections"as emphasized e.g. by Wilder-Smith et al. (2020) . As opposed to SARS 2003 (or the common ‡u), individuals infected with the CoV-2 do not necessarily display symptoms but can nevertheless infect others. There is also …rst evidence (Xing et al., 2020) that even recovered individuals (i.e. those that came from state 2) can carry the CoV-2, also supporting our use of a positive :