Incidence and risk factors for genital Chlamydia trachomatis infection: a 4-year prospective cohort study.

Background: Few long-term studies reporting incidence and behavioral data for Chlamydia trachomatis (CT) infection in the general population have been published. Such studies are important to understand risk factors associated with infection and to develop screening recommendations.

Methods: A fixed prospective 4-year cohort study of 898 sexually active Norwegian women, aged 16 to 23 years at study start, was conducted to assess incidence, repeat infection, and risk factors associated with genital CT infection. Participants were interviewed at study start and at 6-month intervals thereafter for behavioral characteristics. The women were tested for CT infection at 12-month intervals beginning at study start. Risk factors were assessed using Fisher exact test and conditional logistic regression. Person-time was estimated in survival analyses and incidence of CT infection was reported as events per 100 woman-years.

Results: Median duration of observation was 48.0 months (range 10-74) whereas 4.4 specimens were collected per woman (range 2-5). Of the 836 women eligible for the analysis, 19 (2.2%) had a prevalent infection at baseline. The 4-year cumulative incidence of CT infection was 7.7 (95% CI: 6.7-8.7) with annual incidences ranging from 1.2 to 2.9 per 100 woman-years. The 2-year cumulative incidence of repeat CT infection was 11.2 (95% CI: 9.3-13.1) per 100 woman-years. In multivariate analyses, factors associated with incident CT infection were young age (< or =24 years) and number of new partners over the last 12 months prior being tested.

Conclusion: The annual incidences observed for women 24 years or younger with 1 or more new partners over the last 12 months support recommendations for annual testing for CT in this age group in Norway.