Number of sex acts matters for heterosexual transmission and control of Chlamydia trachomatis

Abstract

Mathematical models are instrumental in controlling Chlamydia trachomatis and other sexually transmitted infections (STIs). Modeling approaches that stratify the population by the number of individuals' sex partners often assume the transmission risk per partner to be constant. Sexual behavior data shows, however, that people with many partners share less sex acts per partner than people with fewer partners. This should lower the risk of transmission per partner for highly sexually active individuals and could have important epidemiological consequences for STI transmission and the projected impact of control scenarios. We devise a new epidemiological model that we fit to chlamydia prevalence data from Natsal-2 and CSF, two population-based probability sample surveys of sexual behavior in Britain and France. Compared to a standard model where the transmission risk per partner is constant, a model with realistic numbers of sex acts per partner provides a better fit to the data. Furthermore, the improved model provides evidence for strong assortative mixing among individuals with different numbers of sex partners. Our results suggest that all chlamydia infected individuals with one or more new heterosexual partners per year contribute to ongoing transmission, underlining that control interventions should be aimed towards all sexually active young adults.