Background Age-specific incidence rates of sexually transmitted infections (STI) with Chlamydia trachomatis (CT), Neisseria gonorrhoeae (GC), and Trichomonas vaginalis (TV) are not well characterised in adolescents and young adult women. In this research, we described the CT, GC, and TV incidence rates in young women as functions of age.
Methods Young women aged 14 and 17 were recruited from three adolescent medicine clinics in Indianapolis, Indiana. Study participants were followed longitudinally for up to 8.2 years. Participants were tested for CT, GC, and TV at the time of enrolment, and at subsequent quarterly visits. Infected individuals were treated at or shortly after each visit. We analysed the longitudinally measured incidence infections with CT, GC, and TV using generalised additive mixed effect models (GAMM) with a logit link for binary outcomes. The age effect on STI incidence was modelled as a smooth non-linear function in the GAMM analysis and a random subject effect was included to accommodate the correlations among repeated STI assessments within each individual. Estimated incidence rates (with 95% CIs, in colour) for each organism were reported as smooth functions of age.
Results The cohort included 386 young women. The average age at enrolment of study participants was 15.3 years (SD=1.1 years). A majority (89.1%) of study participants were African-American. The average age at first sexual intercourse was 14.2 years (SD=2, median=14 years). The mean cumulative number of sexual partners measured at the time enrolment was 3 (SD=4, median=2). The average length of follow-up was 3.5 years (SD=2.2 years). Baseline prevalence rates were 10.9%, 4.4% and 6.0% for CT, GC, and TV, respectively. The age-specific incidence rates for the three organisms and any STI are seen in the Abstract P1-S1.12 figure 1.
Conclusions The estimated STI incidence rates clearly differ by organisms, not only in magnitude but also in peak age. While the prevalence rates of the respective organisms in the partner population may be a contributor to the differential risk of STI acquisition, the fact that such differences were observed from the same group of individuals with the same sexual partners and sexual behaviours raises questions about age-related differences in susceptibility to infection by the three organisms.
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