OBJECTIVES: To investigate risk factors for incident and redetected Chlamydia trachomatis (CT) infection in women, including the role of high-risk human papillomavirus (HPV).

METHODS: In this population-based, prospective cohort study conducted in Copenhagen, Denmark, 10,729 women aged 20 to 29 years were tested for CT and HPV DNA and provided information on sexual and health behavior at baseline. Of these, 7998 (74.5%) participated in a follow-up visit 2 years later with identical data collection. We used logistic regression to investigate risk factors for incident and redetected CT infection at follow-up.

RESULTS: Among CT DNA negative women at baseline (n = 7529), 106 (1.4%) were CT DNA positive at follow-up (incident infection). Increasing number of sexual partners during follow-up (odds ratio [OR], 1.07 per partner; 95% confidence interval (CI), 1.02-1.11), low educational level (OR, 1.69; 95% CI, 1.11-2.56; for basic education vs. high school or higher), and high-risk HPV positivity at baseline (OR, 1.66; 95% CI, 1.06-2.58) were risk factors for incident infection, whereas older age (OR, 0.86 per year increase; 95% CI, 0.80-0.93) and condom use (OR, 0.60; 95% CI, 0.38-0.94) were associated with reduced risk. Among CT DNA positive women at baseline (n = 469), 108 (23.0%) tested positive at follow-up (redetected infection). We found no statistically significant associations between age, educational level, sexual behavior, smoking, or high-risk HPV status and the risk for redetected CT.

CONCLUSION: Young age, low educational level, high number of sexual partners, failure to use condoms, and high-risk HPV positivity are associated with increased risk for incident CT infection. These findings may guide the development of targeted CT prevention strategies, including screening and information campaigns.