The unexpected impact of a Chlamydia trachomatis infection control program on susceptibility to reinfection.

BACKGROUND After the introduction of a program to control Chlamydia trachomatis infection in British Columbia, Canada, case rates fell from 216 cases/100,000 population in 1991 to 104 cases/100,000 population in 1997. Since 1998, rates have increased, and case counts now exceed those recorded before the intervention. METHODS We used Cox proportional-hazards survival analysis and developed a compartmental mathematical model to investigate the cause of resurgence in chlamydia cases. RESULTS Cox proportional-hazards survival analysis showed that the relative risk of C. trachomatis reinfection has increased 4.6% per year since 1989, with the increased risk greatest among the young and greater among women than men. A compartmental mathematical model of C. trachomatis transmission showed that a control strategy based on shortening the average duration of infection results in an early reduction in prevalence followed by a rebound in prevalence, reproducing the observed trends. CONCLUSIONS We speculate that a C. trachomatis infection control program based on early case identification and treatment interferes with the effects of immunity on population susceptibility to infection and that, in the absence of strategies to alter sexual networks, a vaccine will be needed to halt the spread of infection at the population level.