Modeling the Impact of Potential Vaccines on Epidemics of Sexually Transmitted Chlamydia trachomatis Infection

Background. We investigated the likely impact of vaccines on the prevalence of and morbidity due to Chlamydia trachomatis (chlamydia) infections in heterosexual populations. Methods. An individual-based mathematical model of chlamydia transmission was developed and linked to the infection course in chlamydia-infected individuals. The model describes the impact of a vaccine through its effect on the chlamydial load required to infect susceptible individuals (the critical load), the load in infected individuals, and their subsequent infectiousness. The model was calibrated using behavioral, biological, and clinical data. Results. A fully protective chlamydia vaccine administered before sexual debut can theoretically eliminate chlamydia epidemics within 20 years. Partially effective vaccines can still greatly reduce the incidence of chlamydia infection. Vaccines should aim primarily to increase the critical load in susceptible individuals and secondarily to decrease the peak load and/or the duration of infection in vaccinated individuals who become infected. Vaccinating both sexes has a beneficial impact on chlamydia-related morbidity, but targeting women is more effective than targeting men. Conclusions. Our findings can be used in laboratory settings to evaluate vaccine candidates in animal models, by regulatory bodies in the promotion of candidates for clinical trials, and by public health authorities in deciding on optimal intervention strategies.