YI7 - Predicting the Potential Impact of a Novel Chlamydia Vaccine in the United States

Background: Chlamydia trachomatis (CT) is a globally prevalent sexually transmitted infection. CT infection is associated with pelvic inflammatory disease and fertility complications in women and men. Novel interventions to control the disease burden of this infection are needed. We investigated the potential impact of novel vaccines against acquisition, transmission, and disease progression of CT infection.

Methods: An age- and sex-structured population-based deterministic mathematical model was developed to describe the impact of vaccination on CT epidemiology in the United States (US). The model stratified the population by age, sex, sexual risk behavior, CT status and stage of infection, and vaccination status. The impact was investigated by varying vaccine efficacy and coverage as well as its duration of protection. The impact was also investigated in priority populations.

Results: Over 10-year horizon to scale up vaccine coverage to 80% in the wider population, a vaccine with 50% efficacy in reducing susceptibility to infection (VE_S=50%), infectiousness (VE_I=50%), or duration of infection (VE_P=50%) resulted, respectively, in 36.0%, 26.4%, and 42.1% reduction in CT prevalence, and 38.6%, 28.5%, and 24.0% reduction in CT incidence rate. If these efficacies acted together, that is VE_S=VE_I=VE_P=50%, CT prevalence and incidence rate were reduced by 66.3% and 60.1%, respectively. After 10-year scale-up, number of vaccinations needed to avert one infection was estimated at 17.7 for VE_S=50%, 25.2 for VE_I=50%, 33.3 for VE_P=50%, and 12.0 for VE_S=VE_I=VE_P=50%. Through age and risk-group prioritization, vaccination was most effective by targeting subpopulations aged 15-19 years and those at highest sexual risk with only 7.7 and 1.8 vaccinations needed to avert one infection, respectively. There were no major differences in impact among women versus men.

Conclusion: A partially efficacious CT vaccine can result in significant reductions in prevalence and incidence of CT infection and its disease burden. Impact is enhanced by targeting vaccination to specific subpopulations.