Taking the widely used antibiotic doxycycline after sex cut the rates of chlamydia and early syphilis in gay, bisexual men and transgender women in San Francisco by half, city health officials announced Monday. The findings offer a glimmer of hope amid a rise in sexually transmitted infections across the country.
This strategy is called doxy-PEP, short for doxycycline post-exposure prophylaxis. In San Francisco, gay, bisexual men, and transgender women with a history of sexually transmitted diseases or multiple sexual partners were given antibiotics and asked to take two 100 mg pills within 72 hours after unprotected sex.
Excluding gonorrhea, new cases of chlamydia and early syphilis have been declining for about a year. The results were presented at the Conference on Retroviruses and Opportunistic Infections in Denver.
“It’s not subtle, it’s very rapid, and we’re seeing the beginning, not the end,” Dr. Hyman Scott, medical director of the San Francisco Department of Public Health, said in an interview. “This is what we want for STI prevention.”
Strategies to eradicate sexually transmitted diseases are urgently needed.
The U.S. Centers for Disease Control and Prevention (CDC) reported in January that syphilis, once nearly eradicated, had reached its highest rate of new infections in the U.S. since 1950. If left untreated, syphilis can damage the heart and brain and cause blindness, deafness, and paralysis.
Chlamydia incidence rates in 2022 were unchanged nationally compared to 2021 figures, but infections were common, with nearly 1.7 million cases. (Gonorrhea cases declined in 2022, but experts cautioned that the trend may be the result of decreased testing.)
“Although these data simply look at the number of STI cases at different points in time, they give us hope that doxy-PEP may reduce STIs at the population level,” said Ina Park, Ph.D., an STI expert at the University of California. San Francisco spoke about the new results. She had no part in it.
Evidence to date supports the case for use of doxycycline only in gay, bisexual men, and transgender women. Previous research has shown that taking a single dose of doxycycline within 72 hours after unprotected sex substantially reduces the risk of sexually transmitted diseases in this group.
“The majority of sexually transmitted diseases in the United States occur in cisgender women,” said Dr. Jonathan Mermin, director of the CDC’s National Center for HIV, Viral Hepatitis, Sexually Transmitted Diseases, and Tuberculosis Prevention.
“Studies on whether doxy-PEP works for cisgender women need to be conducted as soon as possible,” he said.
Last October, the CDC released draft guidance on doxy-PEP. Dr. Mermin said he would issue final recommendations in the coming months.
But spurred by early clinical trial results, the San Francisco Department of Public Health, a pioneer in the fight against HIV and sexually transmitted diseases, began recommending doxy-PEP a year before the agency's draft guidelines were released.
City officials tracked monthly rates of chlamydia, gonorrhea and early syphilis before and after the introduction of doxy-PEP in November 2022. They also compared these figures to infection rates among cisgender women.
Over a 13-month period, new cases of chlamydia and early syphilis across the city fell by 50% compared to expected figures. Gonorrhea incidence rates did not change significantly.
In contrast, chlamydia cases among cisgender women have steadily increased. City health officials plan to analyze the numbers over a long period of time to determine any downward trends.
“The fact that we have not seen a decline in STIs in other populations not recommended for doxy, particularly cis women, leads us to conclude that the decline in cases of chlamydia and early syphilis is related to the rollout of doxy-PEP,” said epidemiologist Madeline Sankaran. “We strengthen it,” he said. San Francisco Department of Public Health announced the findings.
A separate study found that rates of STIs decreased despite an increase in sexual partners, condomless sex, and group sex in people taking doxy-PEP. Researchers found that more than 75% of people who were offered the pill took it without a condom after sex.
Dr Mermin said the results were promising but warned they may have been skewed by the 2022 Mpox outbreak, previously known as monkeypox. During this period, high-risk individuals voluntarily reduced their sexual activity. There was also limited impact on gonorrhea incidence, he added.
Dr. Mermin said that to reduce STIs nationally, this approach must be used in the South and rural areas, as well as in high-risk black and Latino populations.
“No tool, no matter how powerful, can prevent infections if it doesn’t reach the people who need it most,” he said.
There have been concerns that widespread use of doxycycline would lead to the emergence of antibiotic resistance, but the limited data available suggest that this approach does not substantially increase resistance in people taking doxycycline, Dr. Mermin said. He added that the CDC and other agencies are monitoring trends.
If further analysis confirms the potential of doxy-PEP, Dr. Park said, sex party hosts could be expected to provide the drug, just as hospitals currently provide condoms.
Perhaps the antibiotics come in a vial that says, “Take two when you walk out the door,” she said. “To me, that’s where this is happening.”