Objective To examine sources of repeat Chlamydia trachomatis infections using behavioural and molecular methods.

Methods Women with C trachomatis had baseline and 4-month follow-up visits consisting of behavioural surveys and genotyping of C trachomatis. Frequencies and population-attributable risk percentages (PAR%) were estimated for possible sources of repeat infections including sex partners not known to be treated, new sex partners, and sex partners not known to be monogamous. Women with different genotypes at baseline and follow-up were classified as different partner sources of infection.

Results The cumulative incidence of repeat infections in the sample (n=183) was 13% (95% CI 8% to 18%). Predictors of repeat infections included younger age and continued sex with a partner not known to be treated. Frequencies of having partners not known to be treated, new partners, or partners not known to be monogamous at follow-up were 21% (95% CI 15% to 27%), 37% (95% CI 30% to 44%) and 33% (95% CI 28% to 41%), respectively. The PAR% for having a partner not known to be treated was 26% (95% CI 3% to 49%) and for having a new sex partner was 21% (95% CI 0% to 50%). Among eight patients with available genotypes at baseline and follow-up, five had different genotypes and were classified as having a different partner source of infection.

Conclusions Different sex partner sources of repeat C trachomatis infections other than untreated sex partners may contribute substantially to the burden of repeat infections.