where n is the total number of passengers arriving from a given neighboring city; I k, t and N k are the current number of infectious individuals and the total population size of city k, respectively. δ is the delay associated with the duration of each bus trip. The delay δ was calculated as the number of days (rounded down) that a bus, traveling at an average speed of 60 km/h, would take to complete a given trip. The lengths in kilometers of all bus routes were also obtained from the ANTT.