An antibiotic after sex significantly reduced cases of syphilis and chlamydia

A single dose of doxycycline, a widely used antibiotic, taken after sex cut the incidence of chlamydia and early syphilis in half among gay and bisexual men and transgender women in San Francisco, city health officials announced Monday. The findings offered a ray of hope amid a rising wave 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 who had a history of STIs or multiple sexual partners were given a supply of the antibiotic and asked to take two 100-milligram pills within 72 hours of having sex. without protection.

New cases of chlamydia and early syphilis (but not gonorrhea) decreased in 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 curb STIs are urgently needed.

Syphilis, once nearly eliminated in the United States, has reached the highest rate of new infections recorded since 1950, the Centers for Disease Control and Prevention reported in January. If left untreated, syphilis can damage the heart and brain and cause blindness, deafness, and paralysis.

Chlamydia rates remained stable nationwide in 2022, compared to the 2021 figure, but with nearly 1.7 million cases, infections were common. (Gonorrhea cases declined in 2022, but experts warned that the trend could have been the result of a decline in testing.)

“These data simply look at the number of STI cases at different times, but they give me hope that doxy-PEP can reduce STIs at the population level,” said Dr. Ina Park, an STI expert at the University of California. San Francisco said of the new results. She was not involved in the work.

So far, evidence supports the use of 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 sexual intercourse substantially reduces the risk of STIs in these groups.

“The majority of STIs in the United States occur in cisgender women,” said Dr. Jonathan Mermin, director of the CDC’s National Center for HIV, Viral Hepatitis, STD, and Tuberculosis Prevention.

“Studies on whether doxy-PEP works in cisgender women should be implemented as quickly as possible,” she said.

In October, the CDC published draft guidelines for doxy-PEP. It will issue its final recommendations in the coming months, Dr. Mermin said.

But, buoyed by early clinical trial results, the San Francisco Department of Public Health, a pioneer in the fight against HIV and STIs, started to recommend 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 citywide decreased by 50 percent, compared with expected numbers. Gonorrhea rates did not change significantly.

In contrast, chlamydia cases in cisgender women have steadily increased. City health officials plan to analyze the numbers over a longer period to confirm the downward trend.

“The fact that we did not see decreases in STIs in other populations not recommended for doxy, specifically in cis women, reinforces the conclusion that the decrease in cases of chlamydia and early syphilis is related to the implementation of doxy-PEP” said Madeline Sankaran, an epidemiologist. of the San Francisco Department of Public Health, who presented the findings.

A separate study found that STI rates decreased despite an increase in sexual partners, cases of sex without condoms, and group sex among people taking doxy-PEP. More than 75 percent of those offered the pills reported taking them after having sex without condoms, the researchers said.

While the results are promising, Dr. Mermin cautioned, they may have been biased by the 2022 outbreak of mpox, formerly known as monkeypox, during which high-risk groups voluntarily reduced their sexual activity. There was also a limited effect on gonorrhea rates, he added.

Dr. Mermin said reducing STIs nationwide would require ensuring the approach is used in the South and rural areas, and in black and Latino populations who are at high risk.

“No tool, no matter how powerful, will prevent infections if it does not 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 that the approach does not substantially increase resistance in people who take it, Dr. Mermin said. The CDC and others are monitoring trends, he added.

If further analysis confirms the potential of doxy-PEP, Dr. Park said, she could see sex party hosts offering the pills, in the same way that clinics now offer condoms.

Maybe the antibiotics would be in a bottle with a sign that said, “Take two on the way out the door,” he said. “That’s where I see this going.”