In addition to R 0 , a different measurement of the transmission potential using widely available epidemiological data, the actual reproduction number, R a , has been proposed for HIV/AIDS [14] . The concept of R a is much simpler than R 0 in that R a is defined as a product of the mean duration of infectiousness and the ratio of incidence to prevalence [15] . The prevalence p(t) at calendar time t is written as: