Doxy-PEP: population-wide benefits shown in protecting against STIs

By Rebecca Jenkins

Wider use of doxycycline postexposure prophylaxis (doxy-PEP) for bacterial sexually transmitted infections (STIs) in routine clinical care has population-wide benefits, US researchers report.

The doxy-PEP strategy, where individuals take doxycycline (200mg) within 72 hours after condomless sex, had been shown to prevent STIs in randomised clinical trials, but its effect in a real-world setting was largely unknown, the researchers said.

In two analyses published concurrently in JAMA Internal Medicine, US data were analysed to examine the effect of the early roll-out of upscaled doxy-PEP provision in San Francisco, California.

In 2022, San Francisco became the first US jurisdiction to issue guidance on doxy-PEP, recommending it for men who have sex with men (MSM) and transgender women with a history of an STI or multiple partners in the past year.

In one analysis, researchers used San Francisco public health surveillance data to measure monthly cases of bacterial STIs among MSM and transgender women in the two years before the doxy-PEP guidance was introduced compared with the 13-month-period afterwards.

They found cases of early syphilis in the targeted population decreased by 51.39% in the period after the guidelines were implemented, compared with modelled projected cases. Cases of chlamydia reduced by 49.64%, but there was a significant increase in gonorrhoea incidence (25.55%) post-implementation compared with expected rates.

In the other analysis, a retrospective cohort study of 11,551 individuals (95.1% male) receiving HIV pre-exposure prophylaxis in a large Californian health system, pharmacy data showed 2253 (one in five) were dispensed doxy-PEP between 1 November 2022 and 31 December 2023.

Among doxy-PEP recipients, quarterly STI positivity significantly declined by 79% for chlamydia, 80% for syphilis and 12% for gonorrhoea from before to after starting the strategy.

Positivity for STIs remained stable in individuals not dispensed doxy-PEP, the researchers reported.

Dr Vincent Cornelisse, Adjunct Associate Professor at The Kirby Institute, UNSW Sydney, said the latest studies added to randomised trial evidence showing the individual-level benefit of doxy-PEP in preventing syphilis and chlamydia.

‘These studies importantly highlight the potential population-level benefit of large-scale implementation of doxy-PEP among people who are at risk of bacterial STIs,’ he told Medicine Today.

‘As was the case with the earlier randomised trials, the evidence for reduction of gonorrhoea prevalence is less convincing.’

Dr Cornelisse was the principal author of the Australian consensus statement on doxy-PEP (Med J Aust 2024; doi:10.5694/mja2.52258), which recommended clinicians consider prescribing doxy-PEP for a limited time to gay, bisexual and other men who have sex with men (GBMSM) at high risk of syphilis, and perhaps other bacterial STIs, with ongoing review.

‘The statement is more selective than the approach in San Francisco and aims to offer doxy-PEP to people who are at the highest risk of syphilis, while avoiding use by people who might not receive significant benefit,’ Dr Cornelisse said.

He noted that rates of syphilis had risen dramatically in the past decade in Australia among GBMSM, but also in women and their sexual partners, and there was a need to use a range of tools to arrest this trend.

‘However, concerns remain over potential impacts on antibiotic resistance if doxy-PEP were rolled out more broadly, which could emerge both in STI organisms and non-STI organisms,’ he said.

‘So far, antibiotic resistance data from trials are fairly reassuring, but this is an important issue that deserves further attention.’

JAMA Intern Med 2024; doi: 10.1001/jamainternmed. 2024.7178 and doi: 10.1001/jamainternmed.2024.7186.