Background:
Doxycycline post-exposure prophylaxis (doxy-PEP) reduced the risk of chlamydia (CT), gonorrhea (GC) and early syphilis (ES) in MSM and TGW in randomized trials. Little is known about doxy-PEP effectiveness on STI positivity outside of clinical trials.
Methods:
Starting 11/3/22, doxy-PEP was offered to patients receiving PrEP at San Francisco City Clinic who were eligible according to citywide guidelines. We assessed visit-level STI positivity at all visits among MSM and TGW PrEP patients who had at least one clinic visit pre (11/3/21-11/2/22) and post (11/3/22-11/2/23) guideline release. STI positivity was defined for GC and CT as the proportion of visits with a GC or CT test with at least one positive result, and for ES as the proportion of visits with a syphilis test that identified a new ES case. For those who ever initiated (i.e., were prescribed or self-reported use of) doxy-PEP (users), we compared STI positivity at visits pre and post doxy-PEP initiation. For those who did not initiate doxy-PEP (non-users), we compared STI positivity at visits pre and post 11/3/22. We compared the reduction in positivity for users vs. non-users using a generalized estimating equation model with robust variance to account for repeated measures. All data were routinely collected during clinic visits.
Results:
506 patients had visits in both periods: 367 users and 139 non-users. Positivity decreased for CT (RR 0.10, 95% CI 0.05–0.21) significantly in users but not in non-users (RR 0.73, 95% CI 0.44–1.21) (Table); the decrease in positivity was significantly greater among users (p<.0001). ES positivity decreased significantly (RR 0.44, 95% CI 0.21–0.92) in users but not non-users (RR 0.68, 95% CI 0.20–2.30) (Table); these decreases were not significantly different (p=0.56). GC positivity decreased non-significantly for both groups (Table).
Conclusions:
Doxy-PEP significantly reduced positivity for CT and early syphilis among MSM and TGW receiving PrEP at an STI clinic. The difference was significantly greater than the reduction in CT seen in doxy-PEP non-users. There was no significant reduction in GC in doxy-PEP users or non-users. The reasons for reduction in CT positivity and new early syphilis cases in non-users is unclear and warrants further investigation.
