An antibiotic after sex significantly reduced the number of cases of syphilis and chlamydia
A single dose of doxycycline, a commonly used antibiotic, taken after sex halved the incidence of chlamydia and early syphilis among gay and bisexual men and transgender women in San Francisco, city health officials announced Monday. The findings offered a glimmer of hope amid a rising tide of sexually transmitted infections across the country.
The strategy is called doxy-PEP, short for doxycycline post-exposure prophylaxis. In San Francisco, gay and bisexual men and transgender women with a history of STDs or multiple sex partners were given a supply of antibiotics and asked to take two 100-milligram pills within 72 hours of unprotected sex.
The number of new cases of chlamydia and early syphilis – but not gonorrhea – fell over about a year. The results were presented at the Conference on Retroviruses and Opportunistic Infections in Denver.
“It’s not subtle, it’s happening very quickly and we’re seeing the beginning of it, not the end,” said Dr. Hyman Scott, medical director of the San Francisco Department of Public Health, in an interview. “This is what we want for STD prevention.”
Strategies to combat STDs are desperately needed.
Syphilis, once nearly eradicated in the United States, now is reached the highest number of new infections since 1950The Centers for Disease Control and Prevention reported this in January. If left untreated, syphilis can damage the heart and brain and cause blindness, deafness and paralysis.
Chlamydia cases remained flat nationwide in 2022 compared to 2021, but infections were common at nearly 1.7 million cases. (Gonorrhea cases fell in 2022, but experts warned the trend may have been the result of a decline in testing.)
“These data simply look at the number of STD cases at different points in time, but give me hope that doxy-PEP can reduce STDs on a population level,” says Dr. Ina Park, an STD expert at the University of California. San Francisco, said of the new results. She was not involved in the work.
The evidence to date supports the case for using doxycycline only in gay and bisexual men and transgender women. Previous studies have shown that a single dose of doxycycline taken within 72 hours of unprotected sex significantly reduces the risk of STDs in these groups.
“The majority of STDs in the United States occur in cisgender women,” said Dr. Jonathan Mermin, director of the National Center for HIV, Viral Hepatitis, STD and TB Prevention at the CDC.
“Studies on whether doxy-PEP works in cisgender women should be implemented as soon as possible,” he said.
In October, the CDC released it design guidelines for doxy-PEP. It will issue its final recommendations in the next few months, said Dr. Mermin.
But, following the results of early clinical trials, the San Francisco Department of Public Health – a pioneer in the fight against HIV and STDs – started recommending doxy-PEP a year before the agency’s draft guidelines were published.
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 the numbers to the rate of infections in cisgender women.
Over a 13-month period, new cases of chlamydia and early syphilis in the city fell by 50 percent, compared to expected numbers. The number of cases of gonorrhea did not change significantly.
In contrast, chlamydia cases in cisgender women increased steadily. City health officials plan to analyze the numbers over a longer period of time to confirm the downward trend.
“The fact that we have not seen a reduction in STDs in other populations not recommended for doxy, particularly cis women, strengthens the conclusion that the reduction in chlamydia and early syphilis cases is related to the rollout of doxy PEP,” said Madeline Sankaran, an epidemiologist. at the San Francisco Department of Public Health, who presented the findings.
A separate study found that rates of STDs fell despite an increase in the number of sexual partners, instances of sex without condoms and group sex among people taking doxy-PEP. More than 75 percent of those offered the pills reported taking them without condoms after sex, the researchers said.
Although the results are promising, Dr. cautioned. Mermin, they may be skewed by the 2022 outbreak of mpox, formerly known as monkeypox, in which at-risk groups voluntarily limited their sexual activity. There was also a limited effect on the number of gonorrhea cases, he added.
Dr. Mermin said reducing STDs nationwide should ensure the approach is applied in the South and rural areas, and among Black and Latino populations at high risk.
“No tool, no matter how powerful, can prevent infections if it doesn’t reach the people who need it most,” he said.
There has been some concern about the emergence of antibiotic resistance with the widespread use of doxycycline, but the limited data available suggest the approach does not substantially increase resistance in people who use it, said Dr. Mermin. The CDC and others are monitoring the trends, he added.
If further analysis confirms doxy-PEP’s potential, Dr. Park said, she could foresee sex party hosts offering the pills, just as clinics now offer condoms.
Maybe the antibiotics were in a jar with a sign that said, “Take two with you when you leave the house,” she said. “That’s where I see this happening.”