Implementing HIV treatment as prevention (TasP) may potentially influence risk behavior among key groups such as men who have sex with men (MSM). We measured the incidence and determinants of chlamydia (CT), gonorrhea (GC) and syphilis infection within a prospective cohort of MSM in Vancouver, Canada where TasP has been policy since 2010.
Eligible participants were recruited from 2012-2015, aged ≥16 years of age and reported recent sex with another man. Participants completed study visits every 6 months, which included a computer-assisted self-interview on demographics, sexual behavior and substance-use, and a nurse-administered clinical questionnaire. A rapid HIV test was administered and venous blood sample taken for serology for syphilis. Urine NAAT tests and/or pharyngeal, rectal or urethral swabs for chlamydia and gonorrhea were offered as an optional service. We used generalized estimating equations to identify factors associated with any incident STI infection, either diagnosed through a study visit or reported between study visits by the participant. A multivariable model was built using backward selection, Type III p-values (p<0.05), and QIC minimization.
Of 575 MSM (29.4% HIV-positive and 70.6% HIV-negative at enrollment) with follow-up, 134 (23.3%) had an incident STI. Prior STI diagnosis was strongly associated with an incident STI (relative risk [RR]: 25.07, 95% CI: 19.03-33.03, p<0.001). During a median of 1.98 person-years of follow-up 77 chlamydia, 69 gonorrhea, and 37 syphilis cases were reported/diagnosed, for an incidence rate of 7.14 per 100 person-years (PYRs) for CT, 6.40 per 100 PYRs for CT and 3.43 per 100 PYRs for syphilis. Any STI incidence did not differ by HIV status (p=0.85). Factors independently associated with incident STI were younger age (adjusted RR [aRR]=0.98 per year older; 95% CI: 0.96-0.99), group sex event attendance (aRR=1.49, 95% CI: 1.08-2.07), anal sex with casual partners (aRR=2.78, 95% CI: 1.87-4.14), poppers use (aRR=1.61, 95% CI: 1.16-2.22), and injection drug use behavior (aRR=1.89, 95% CI: 1.20-2.97). Further, MSM who reported only having condomless anal sex with HIV-positive men on treatment or with low viral loads were more likely to have an incident STI infection (aRR=1.40, 95% CI: 0.98-1.98).
STI incidence and re-infection was common. This may be a partial concomitant effect of TasP scale-up, and may reduce some of its benefits without an additional focus on primary prevention of other STIs.