Julia Marcus from Harvard Medical School and colleagues from Australia and the United States conducted a retrospective cohort study and found that post-exposure prophylaxis with doxycycline sharply reduces the incidence of sexually transmitted infections (STIs) among people receiving pre-exposure prophylaxis for HIV. The analysis included more than 11,500 adults (mean age 39.9 years; 95.1 percent men) who received pre-exposure prophylaxis for HIV in the Northern California health care system from November 2022 to December 2023. 2,253 (19.5 percent) of them received doxycycline for post-exposure prophylaxis, 1,096 of whom were infected with STIs during the previous year. The incidence of positive tests for these infections was assessed quarterly for a year from the start of post-exposure prophylaxis. The results are published in the journal JAMA Internal Medicine.
It was found that with post-exposure prophylaxis with doxycycline, the quarterly rate of positive tests for chlamydia decreased from 9.6 to 2.0 percent; relative risk (RR) 0.21 (p < 0.001), and the decrease was observed for all localizations of infection. The same indicator for gonorrhea decreased from 10.2 to 9.0 percent (RR 0.88; p = 0.048), significantly for lesions of the rectum (RR 0.81; p = 0.02) and urethra (RR 0.56; p = 0.001), but not pharynx. For syphilis, the quarterly rate of positive tests decreased from 1.7 to 0.3 percent (RR 0.20; p < 0.001). Among those who did not receive post-exposure prophylaxis, these indicators remained stable. According to the authors of the study, this indicates the potential benefit of such prevention in reducing STI levels in wider segments of the population.