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 d₁ years shortly after infection, the frequency g₁ is very high. Subsequently, for d₂ years (i.e., during the asymptomatic period), g₂ is persistently low, followed by a time period with high infectiousness g₃ for d₃ years until death or no secondary transmission due to AIDS. Following a statistical study [20], d₁, d₂ and d₃ are assumed to be 0.24, 8.38 and 0.75 years. Assuming that the contact frequency does not vary as a function of time since infection, g₁, g₂ and g₃ are estimated at 1.30, 0.05 and 0.36 per year.