The probability that an infected individual arrives at any location in the worldwide air-transportation network (S) is the product of 2 quantities: 1.) The probability that a patient boards a plane at an outbreak location (Q) multiplied by the probability that the patient takes one among very many possible routes to a destination and gets off the plane (P):
S = Q x P
If, for example, the first probability is Q=1/10000 and the second probability is P=1/10, the total probability is S=1/100000. The first component of the probability is difficult to estimate, the second part can be computed using a computational model for movement patterns on the worldwide air-transportation network. This second quantity (P) is called the relative import risk: Given that an infected individual boards a plane at airport X, what is the probability that the person arrives at point Y.
It is important to understand that the absolute (actual) import risk is much much lower than the relative import risk. The relative risk is only
useful for comparing different locations and alone does not predict the import probability at a certain destination.