DoxyPEP effective for preventing syphilis, chlamydia
Two studies of early uptake of doxycycline postexposure prophylaxis (doxyPEP) found decreases in sexually transmitted infections at both the population and the clinic level.
Doxycycline postexposure prophylaxis (doxyPEP) was effective for prevention of bacterial sexually transmitted infections (STIs) in two recent studies.
DoxyPEP involves 200 mg of doxycycline to be taken within 72 hours of unprotected sex. In a retrospective cohort study of adults who received HIV preexposure prophylaxis (PrEP) from Nov. 1, 2022, to Dec. 31, 2023, researchers assessed demographic and clinical characteristics of patients who were dispensed doxyPEP and their rates of bacterial STIs. The study's primary outcomes were incident chlamydia, gonorrhea, or infectious syphilis measured as quarterly STI positivity (the proportion of patients who tested positive at least once per quarter). Rate ratios (RRs) in patients who received doxyPEP were used to compare mean quarterly STI positivity from 24 months before to 12 months after doxyPEP was started. STI trends for the full cohort were evaluated in an exploratory analysis stratified by receipt of doxyPEP. The results were published Jan. 6 by JAMA Internal Medicine.
The study included 11,551 patients who took HIV PrEP (mean age, 39.9 years; 95.1% male). Of these, 2,253 (19.5%) were dispensed doxyPEP, most of whom (98.9%) were male and 48.6% of whom had an STI in the year before doxyPEP was started. Those who were dispensed doxyPEP were older (mean age, 40.4 vs. 39.8 years; P=0.04), had used HIV PrEP longer (mean, 4.2 vs. 3.4 years; P<0.001), and were more likely to be commercially insured (92.8% vs. 88.9%; P<0.001) than those who did not receive it.
Among those who received doxyPEP, quarterly chlamydia positivity decreased from 9.6% (95% CI, 9.0% to 10.3%) before receipt to 2.0% (95% CI, 1.5% to 2.6%) afterward (RR, 0.21 [95% CI, 0.16 to 0.27]; P<0.001); significant decreases were seen for each anatomic site of infection. For gonorrhea, quarterly positivity decreased from 10.2% (95% CI, 9.6% to 10.9%) before doxyPEP to 9.0% (95% CI, 8.0% to 10.1%) afterward (RR, 0.88 [95% CI, 0.77 to 1.00]; P=0.048), with significant site-specific declines for rectal and urethral gonorrhea, but not pharyngeal gonorrhea. For syphilis, quarterly positivity decreased from 1.7% (95% CI, 1.4% to 1.9%) before doxyPEP to 0.3% (95% CI, 0.2% to 0.6%) afterward (RR, 0.20 [95% CI, 0.11 to 0.37]; P<0.001). Rates of STI positivity did not change in patients who were not dispensed doxyPEP.
Among other limitations, the researchers noted that the early adopters of doxyPEP in their study may not be representative of the overall population who may use or would benefit from the regimen, that they had no data on gender identity or sexual orientation, and that they could not prove a causal association between doxyPEP and STI incidence. “In this cohort study, among individuals accessing HIV PrEP, receipt of doxyPEP was associated with substantial declines in chlamydia and syphilis incidence and moderate, site-specific declines in gonorrhea incidence during the first year of doxyPEP availability,” the authors wrote. “DoxyPEP may offer substantial benefits for reducing population-level STI transmission with broader implementation.”
The second study, also published Jan. 6 by JAMA Internal Medicine, examined whether doxyPEP was associated with a citywide decrease in STIs in San Francisco. The analysis measured monthly cases of chlamydia, gonorrhea, and early syphilis before (July 2021 to October 2022) and after (November 2022 to November 2023) citywide doxyPEP guidelines were released in October 2022.
There were 6,694 cases of chlamydia, 9,603 cases of gonorrhea, and 2,121 cases of early syphilis in men who have sex with men and in transgender women during the analytic period.
Cases of chlamydia (−6.58% per month; 95% CI, −7.99% to −5.16%) and early syphilis (−2.68% per month; 95% CI, −3.75% to −1.60%) decreased significantly versus model projections after doxyPEP implementation. Infections had decreased by 49.64% (95% CI, −59.05% to −38.06%) and 51.39% (95% CI, −58.21% to −43.46%), respectively, compared with projected cases after 13 months. Monthly gonorrhea cases, however, increased significantly compared with projections (1.77% per month; 95% CI, 0.87% to 2.67%).
The researchers noted that they could not prove causality and that they had limited data on doxyPEP use in San Francisco during this period, among other limitations. They concluded that their analysis shows the potential for substantial population-level reductions in STIs with doxyPEP in men who have sex with men and transgender women but said that data to more broadly characterize doxyPEP prescriptions and adherence are needed. “Meanwhile, supporting doxyPEP implementation through clinician and patient education and ensuring low-barrier access to doxyPEP for [men who have sex with men] and transgender women with a greater likelihood of STIs could have a significant impact on the nationwide epidemic of STIs,” they wrote.
An editorial accompanying both studies said they “provide welcome evidence for the effectiveness of doxyPEP in preventing syphilis and chlamydia acquisition among an early-adopter group” and “direction for the next generation of studies.” Future studies could compare doxyPEP intake to efficacy, examine sustained efficacy, and evaluate the impact of doxyPEP on antimicrobial resistance trends, the editorialists said. “The evolving database will be necessary to assess long-term risks and benefits in other populations and to track progress as the targeted pathogens inevitably evolve,” they wrote. “Research to identify and test new and improved STI therapeutic and prevention options, especially vaccines and antimicrobials, with safety and efficacy in all populations remains paramount.”