Syphilis and chlamydia infections were reduced by half among men who have sex with men and transgender women 1 year after San Francisco rolled out doxycycline postexposure prophylaxis (doxy-PEP), according to data presented at the Conference on Retroviruses and Opportunistic Infections (CROI) this week.
After a clinical trial showed that doxy-PEP taken after sex reduced the chance of acquiring syphilis, gonorrhea, and chlamydia by about two-thirds, the San Francisco Department of Public Health released the first guidelines in the country in October 2022.
So far, more than 3700 people in San Francisco have been prescribed doxy-PEP, reports Stephanie Cohen, MD, director of HIV and sexually transmitted infection (STI) prevention in the Disease Prevention and Control Branch of Public Health.
Dr. Cohen and her colleagues spent a year monitoring the uptake of doxy-PEP and used a computer model to predict what the rates of sexually transmitted infection would have been without doxy-PEP.
In November 2023, 13 months after the guidelines were introduced, they found that monthly chlamydia and early syphilis infections were 50% and 51% lower, respectively, than what was predicted by the model.
Fewer Infections
The drop in infections is having a tangible effect on patients in San Francisco, and many clinicians are noting that they are seeing far fewer positive tests. “The results that we’re seeing on a city-wide level are absolutely being experienced by individual providers and patients,” Dr. Cohen said.
However, the analysis showed no effect on rates of gonorrhea. It’s not clear why, although Dr. Cohen points out that doxy-PEP was less effective against gonorrhea in the clinical trial. And “there could be other factors in play,” she added. “Adherence might matter more, or it could be affected by the prevalence of tetracycline resistance in the community.”
With rates of STIs, particularly syphilis, quickly rising in recent years, healthcare providers have been scrambling to find effective interventions. So far, doxy-PEP has shown the most promise. “We’ve known for a while that all of the strategies we’ve been employing don’t seem to be working,” noted Chase Cannon, MD, an infectious disease specialist at the University of Washington in Seattle. “That’s why doxy-PEP is important. We haven’t had anything that can deflect the curve in a long time.”
What About the Side Effects?
Some concerns remain, however, about the widespread prophylactic use of antibiotics. There are no long-term safety data on the potential side effects of doxy-PEP, and there is still a lot of stigma around interventions that allow people to have sex the way they want, said Dr. Cannon.
But perhaps, the biggest concern is that doxy-PEP could contribute to antibiotic resistance. Those fears are not misplaced, Dr. Cannon added. The results of one study, presented in a poster at CROI, showed that stool samples from people prescribed doxy-PEP had elevated levels of bacterial genes that can confer resistance to tetracyclines, the class of antibiotics to which doxycycline belongs. There was no change in resistance to other classes of antibiotics and no difference in bacterial diversity over the 6 months of the study.
Dr. Cannon cautioned, however, that we can’t extrapolate these results to clinical outcomes. “We can look for signals [of resistance], but we don’t know if this means someone will fail therapy for chlamydia or syphilis,” he said.
There are still many challenges to overcome before doxy-PEP can be rolled out widely, Dr. Cohen explained. There is a lack of consensus among healthcare professionals about who should be offered doxy-PEP. The clinical trial results and the San Fransisco guidelines only apply to men who have sex with men and to transgender women.
Some clinicians argue that the intervention should be provided to a broader population, whereas others want to see more research to ensure that unnecessary antibiotic use is minimized.
So far just one study has tested doxy-PEP in another population — in women in Kenya — and it was found to not be effective. But the data suggest that adherence to the protocol was poor in that study, so the results may not be reliable, Dr. Cohen said.
“We need effective prevention tools for all genders, especially cis women who bear most of the morbidity,” she said. “It stands to reason that this should work for them, but without high-quality evidence, there is insufficient information to make a recommendation for cis women.”
The US Centers for Disease Control and Prevention is currently reviewing public and expert comments and refining final guidelines for release in the coming months, which should alleviate some of the uncertainty. “Many providers are waiting for that guidance before they will feel confident moving forward,” Dr. Cohen noted.
But despite the risks and uncertainty, doxy-PEP looks set to be a major part of the fight against STIs going forward. “Doxy-PEP is essential for us as a nation to be dealing with the syphilis epidemic,” Carl Dieffenbach, PhD, director of the Division of AIDS at the National Institute of Allergy and Infectious Disease, said in a video introduction to CROI.
A version of this article appeared on Medscape.com.