where D is the mean generation time (or what was previously described as the mean duration of infectiousness [14, 15] ). If the transmission rate () is constant  (i.e., independent of infection-age), R a = D. Moreover, from equation (2), R 0 is given by: Figure 1 . The relative frequency of secondary transmissions of HIV as a function of the time since infection. A step function is employed to approximately model the frequency of secondary transmissions relative to infection-age. For d 1 years shortly after infection, the frequency g 1 is very high. Subsequently, for d 2 years (i.e., during the asymptomatic period), g 2 is persistently low, followed by a time period with high infectiousness g 3 for d 3 years until death or no secondary transmission due to AIDS. Following a statistical study [20] , d 1 , R a coincides with R 0 as long as () is constant. Nevertheless, in many instances, the contact frequency and infectiousness (which may be partly reflected, for example, in the viral load distribution of the infected host) vary as a function of infection-age . The variation in () is particularly the case for HIV infection. Thus, although the usefulness of the incidence-to-prevalence ratio and R a has been emphasized to have an application in HIV/AIDS [15] , R a tends to yield a biased estimate (if R a is regarded as a proxy for R 0 ), and the estimate of R a is not as robust as that is obtained with equation (5) to objectively interpret the transmission potential [17, 18] .