Although it is possible that some sick individuals may change their behavior due to the symptoms [15] , we assume that sick individuals will not change their behavior and continue to have the same number of daily contacts as a healthy individual. Therefore, we set the baseline value for the reduced number of contacts due to illness h at 1, as people usually do not greatly alter their daily behavior during the incubation period. Individuals in the exposed classes, E and E m , are thought to be 50% less infectious due to incubation than those in the infected classes, I and I m , so we set a = 0.5 [19, 31] .