428. Lower Incidence Rates of Neisseria gonorrhoeae and Chlamydia trachomatis in Pre- Exposure Prophylaxis Patients Over Fifty Years Old Than in Younger Quartiles.

BACKGROUND: While Pre-Exposure Prophylaxis (PrEP) has been shown to be effective in preventing the acquisition of HIV, the cost of care is considerable. Public health efforts should be directed to those at highest risk to minimize the cost of a quality-adjusted life-year (QALY). Given that sexually transmitted illnesses (STIs) increase the risk of acquiring HIV from 5–8 fold, their incidence rates can serve to define populations at highest risk for HIV within the community of men who have sex with men (MSM). METHODS: A retrospective cohort of PrEP patients with STI screening visits in two clinics in Dallas, Texas from January 2016 through April 2019 were reviewed. Three-site genital and extra-genital testing was routinely performed. Categorical variables were compared using Chi-Square. A binomial regression model was used to evaluate the odds of STI acquisition. RESULTS: 201 PrEP patients had 999 screening visits. 17 were in the first quartile, ages (18–24) 35.3% were Black, 70.6% MSM; 105 were in the second quartile, ages (25–37) 23.8% Black, 58.1% MSM; 56 third quartile, ages (38–49) 17.9% Black, 62.5% MSM; 23 patients in the fourth quartile, age ≥50, 8.7% were Black, 47.8% MSM. The younger age group, first quartile contributed 16 person-years (py); the second quartile, 102 py; third quartile, 48 py; fourth quartile, ≥50, 25 py. The incidence rates for Neisseria gonorrhoeae (GC) or Chlamydia trachomatis (CT) from any site were higher in the first quartile, 166/100 py, CI 150–174; second quartile, 92/100 py, CI- 85–96; third quartile, 62/100 py, CI 48–75 than in those in the fourth quartile, ≥50 years old, 44/100 py, CI 27–63. The incidence of any rectal GCor CT in the first quartile, 86/100 py, CI 63–98; second quartile, 53/100 py, CI 43–62; third quartile, 29/100 py, CI 18–43 and those in the fourth quartile, ≥50, 32/100 py, CI 17–52. In a binomial regression model, the odds of acquiring any GC or CT for a PrEP patient ≥50 revealed an adjusted OR of 0.40, CI, 0.20–0.79 (adjusted for race and sexual orientation) P = 0.009 when compared with patients less than 50. CONCLUSION: PrEP patients fifty and older have the lowest odds of acquiring STIs. Younger age groups with higher incidences of STIs may represent those with more episodes of condomless sex, at higher risk for HIV. DISCLOSURES: All authors: No reported disclosures.