(see Text S1 for a derivation). The first factor in equation ( Equation (1) shows that the pathogen cannot invade the population if the rate of antiviral prophylaxis exceeds a certain critical rate of antiviral prophylactic therapy s c , which is given by the solution of the equation R 0 = 1. For the default parameter values (Table 1 ) it turns out that the critical rate of antiviral prophylaxis is s c = 1.96 (yr 21 ) if antiviral drugs provide complete protection against infection (AVE S = 1). This implies that approximately two-thirds of the population needs to be protected against infection by antiviral prophylaxis in order to prevent an epidemic. This fraction increases if antiviral drugs provide partial protection against infection and subsequent transmission.