Background Chlamydia trachomatis (CT) infection remains highly prevalent. CT reinfection occurs in up to 20% of persons within months after treatment, likely contributing to sustaining the high chlamydia prevalence. Most studies evaluating predictors of reinfection have focused on epidemiological and behavioural factors. Our program is studying host immune responses and organism factors contributing to reinfection. In this study, we evaluate the influence of CT organism load on reinfection risk.

Methods In an ongoing study, women presenting to an STD Clinic in Birmingham, AL, for CT infection treatment are enrolled, treated, and return for 3- and 6-month follow-up visits. At each visit, clinical information and endocervical swabs are collected. CT detection and organism load quantification is performed using real-time PCR. To estimate organism load, a CT calibrator is run using stock CT samples with known organism counts to create cycle threshold standard curves for comparison with clinical samples, providing reliable and reproducible results that allow for relative quantification on a log scale.

Results Of 119 participants completing the study to date: 95% were African American and 56% had prior CT infection (per report and chart review). The median log10 CT load at enrollment was 5.8/mL (range 3.1 – 8.9). CT reinfection occurred in 22 (18%). The median log10 load at enrollment was significantly lower in those with subsequent reinfection compared with those without reinfection (5.05/mL vs. 6.1/mL; P = 0.012 by Wilcoxon rank sum test).

Conclusion A lower endocervical CT organism load at the time of treatment was associated with a greater CT reinfection risk. The reason for this is unclear, but it is possible lower organism loads could elicit weaker protective cellular immune responses, predisposing to greater reinfection risk. In addition to continuing organism load testing on more samples to verify this association, we will be investigating cellular immune responses in this cohort.

Disclosure of interest statement Nothing to Declare.