Schematic overview of the chlamydia infection process in a pair model that allows for reinfection in ongoing partnerships (A) and in a standard model in which partnerships are instantaneous and reinfection is not incorporated (B). In the pair model, a compartment consists of a pair of 2 individuals which can be susceptible (S), infected (I), or recovered (R). The pair formation process is independent of the infection process and is therefore not shown in this figure. Transmission can only occur in a pair including a susceptible and an infected individual (SI) with rate βϕ (β denotes the transmission probability per sex act and ϕ the frequency of sex acts, indicated in red). After chlamydial infection, an individual can clear the infection naturally (with rate ε per year, indicated in black). After natural clearance, an individual is immune for a duration of 1/γ years (indicated in green). An individual can also be screened and treated (with rate α per year) and can notify his or her partner with probability q (indicated in blue). The partner always accepts treatment. After treatment no immunity is assumed. In the standard model, the transmission parameter λ is a combination of the number of new partners per year and the transmission probability per partnership.

Impact of screening men and women for 5 consecutive years on chlamydia prevalence. A, Chlamydia prevalence in a pair model that allows for reinfection in the absence (gray line) or presence of partner notification (PN) (gray dashed line) and in the standard model (black line). B, Difference in the estimated relative impact of screening of the 2 models in the absence of PN (black line) and in the presence of different levels of PN (gray lines; lighter gray indicates higher levels of PN). The straight black line shows the point of equality, the level of PN needed in the pair model that allows for reinfection to estimate the same impact of screening as the standard model. C, Point of equality for different assumptions of the duration of immunity after natural clearance of chlamydia for a screening rate of 0.2/year. For all levels of immunity, the transmission probability per sex act (pair model) and the transmission parameter (standard model) is calibrated to a 3.5% baseline prevalence. The scenario used in A and B is indicated with a black dot.