Incidence and Duration of Pharyngeal Chlamydia Among a Cohort of Men Who Have Sex With Men

Background The prevalence of pharyngeal chlamydia is low, but its incidence and duration are unknown. A high incidence or duration may support the role of pharyngeal chlamydia in sustaining chlamydia transmission. Methods From March 2016 to December 2018, we enrolled men who have sex with men (MSM) in a 48-week cohort study in Seattle, Washington. Participants self-collected pharyngeal specimens weekly. We tested specimens using nucleic acid amplification testing at the conclusion of the study. In primary analyses, we defined incident pharyngeal chlamydia as >2 consecutive weeks of a positive pharyngeal specimen. In sensitivity analyses, we defined incident chlamydia as >1 week of a positive specimen. We estimated duration of pharyngeal chlamydia, censoring at loss to follow-up, receipt of antibiotics, or end of study. Results A total of 140 participants contributed 70.5 person-years (PY); 1.4% had pharyngeal chlamydia at enrollment. In primary analyses, there were 8 pharyngeal chlamydia cases among 6 MSM (incidence = 11.4 per 100 PY; 95% confidence interval [CI]: 6.0-21.9). In sensitivity analysis, there were 19 cases among 16 MSM (incidence = 27.1 per 100 PY; 95% CI: 18.5-39.8). The median duration was 6.0 weeks (95% CI: 2.0-undefined) in primary analysis and 2.0 weeks (95% CI: 1.1-6.0) in sensitivity analysis. Duration was shorter for those with a history of chlamydia compared with those without (3.6 vs 8.7 weeks; P = .02). Conclusions Pharyngeal chlamydia has a low incidence and duration relative to other extragenital sexually transmitted infections. Its contribution to population-level transmission remains unclear. In this 48-week natural history study of pharyngeal chlamydia among men who have sex with men, we observed an incidence of 11.4 per 100 person-years and estimated the median duration of infection to be 6 weeks.