P1-S5.41 Quantifying the contribution of re-infection within partnerships to persistent spread of Chlamydia

Abstract

Background Many studies show that people testing positive and treated for genital chlamydia infection are at high risk of a repeat infection in the first year after treatment. Repeat infection can come from treatment failure, a new partner, or from within an existing partnership if the partner was not treated. Partnerships can therefore form a reservoir of chlamydia infections that might affect the effectiveness of population screening programs. It is not known how much reinfection within partnerships contributes to the persistent spread of chlamydia in heterosexual populations. Methods We derive an expression for the basic reproduction number of an SIS infection in a population with long term monogamous partnerships. The basic reproduction number contains an explicit term quantifying the contribution of re-infection within partnerships to the overall reproduction number. The derivation is then extended to include partner notification and treatment. Finally, the model is extended to include two types of partnerships with differing average duration. Results For small recovery rates and low transmission probabilities reinfection plays a minor role in sustaining chlamydia transmission. However, there is an optimal combination of duration of infection and transmission probability for which reinfection contributes substantially to keeping chlamydia endemic in a population. We discuss the functional dependency of the basic reproduction number on these parameters. Using a more complex model numerical scenarios were simulated showing that partner notification prevents a large proportion of re-infections. Conclusions The effect of screening depends, in part, on whether or not it succeeds in moving the basic reproduction number away from the transmission optimum via reinfection. This can be achieved either by choice of the screening interval or by rescreening those individuals who tested positive in a first screening test. There is an optimal time interval for retesting that minimises the basic reproduction number. The precise numerical value depends on partnership durations and transmission probabilities.