Figure 1.

Illustration of the model. The 4 health states are mutually exclusive, and transition occurs when rectal CT/GC infection is acquired, when rectal CT/GC infection is cleared, or when HIV is acquired. The HIV incidence rate among those without rectal CT/GC infection is λ_HIVΩ_HIV, where λ_HIV is the HIV incidence at the onset of the screening program and Ω_HIV is an adjustment factor to account for changes in the prevalence of HIV in sex partners over time. The rate of HIV incidence among those with rectal CT/GC infection is assumed to be ϴ times that of those without rectal CT/GC infection. The incidence rate of rectal CT/GC infection is λ_STDΩ_STD and ${\widehat{\lambda }}_{\text{STD}}{\Omega }_{\text{STD}}$ among those without and with HIV, respectively, where λ_STD and ${\widehat{\lambda }}_{\text{STD}}$ are the incidence rates of rectal CT/GC infection at the onset of the screening program among those without and with HIV, respectively, and Ω_STD is an adjustment factor to account for changes in the prevalence of CT and GC in sex partners over time as a result of the screening program. The clearance rate of rectal CT/GC infection (r) is a function of 2 factors: duration of infection and screening rates.