Background Chlamydia trachomatis (C. trachomatis) is the most common sexually transmitted bacterial infection among young individuals, and Chlamydia transmission is closely related to sexual behaviour. Previous studies have found overlap between sexual partnerships to be a possible predictor of C. trachomatis, but the importance of this finding has only been briefly described when other significant predictors are also taken into account.

Aim The aim of the study was to examine the importance of overlap between sexual partnerships as a predictor of C. trachomatis infection in young Danes aged 15–29 years.

Method Detailed sexual behaviour data were collected via a web-based questionnaire. A total of 1,445 young Danes with a laboratory-verified C. trachomatis infection registered in the Central Denmark Region and 1,000 randomly selected Danes (control group) aged 15–29 years were invited to participate. Using multivariate analysis, we calculated the odds ratio (OR) for having overlap between sexual partnerships in the group of C. trachomatis infected individuals as compared with the controls.

Results The participation rates were 15.5% (217/1,445) among the C. trachomatis infected and 35.6% (348/1,000) in the general population. In the multivariate analysis, we found that young Danes diagnosed with a C. trachomatis infection were more likely to have had concurrent partnerships than individuals in the control group.

• At least 1 overlap between sexual partners; Adj. OR = 10.1 (95% CI: 4.8–21.1)

• At least 2 overlaps between sexual partners; Adj. OR = 0.6 (95% CI: 2.2–51.1)

• At least 3 overlaps between sexual partners; Adj. OR = 9.9 (95% CI: 2.0–48.4)

Conclusion Concurrent sexual partnerships are an important predictor for C. trachomatis infection in young adults aged 15–29 years when other significant predictors are taken into account.