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Rates of sexually transmitted infections (STIs) went higher among men after they began pre-exposure prophylaxis (PrEP) to prevent HIV infection, Australian researchers found.

Among patients (most of them gay or bisexual) with pre-enrollment data, incidence of STIs rose from 69.5 per 100 person-years prior to enrollment to 98.4 per 100 person-years during follow-up, reported Michael W. Traeger, of the Burnet Institute in Melbourne, and colleagues.

After adjusting for testing frequency, significant increases in incidence of any STI (adjusted incidence rate ratio 1.12, 95% CI 1.02-1.23) and chlamydia (adjusted IRR 1.17, 95% CI 1.04-1.33) were seen, the authors wrote in .

This was an exploratory outcome of the study, whose primary outcome was the incidence of any STI during the follow-up period in all participants (including those without pre-enrollment data) -- a rate of 91.9 per 100 person-years, they noted.

The rising rates of STIs linked to PrEP use has become a controversial topic. One recent article in the U.S. edition of suggesting that PrEP may be "propelling" the epidemic of STIs, citing a by Traeger's group that found PrEP use was linked with a significant increase in rectal chlamydia and any STI diagnosis.

However, Traeger and colleagues noted here that "many studies have lacked comprehensive data for STI incidence prior to use of PrEP, and only a few have accounted for changes in population STI rates or for the confounding effect of increased STI testing."

Researchers examined a secondary study objective from the (PrEPX) study, whose primary objective was "to measure changes in population-level HIV incidence in Victoria, Australia following study rollout." It looked at 2,981 participants in 5 Australian Collaboration for Coordinated Enhanced Sentinel Surveillance (ACCESS) clinics, who had at least one follow-up visit.

Median age of participants was 34, nearly all were men and about 95% were gay or bisexual. The primary outcome was incidence of chlamydia (n=1,434), gonorrhea (n=1,242), or syphilis (n=252). Overall, 2,928 STIs were diagnosed in 48% of participants. The authors found that a quarter of the participants accounted for about three-quarters of all STIs.

An exploratory analysis of 2,058 participants with complete data found certain factors associated with greater risk of STIs:

• Younger age
• Diagnosis of rectal chlamydia, gonorrhea, or syphilis prior to enrollment
• Greater number of anal sex partners
• Participation in group sex

However, they noted that "inconsistent or no condom use with casual sex partners" was not linked with risk of STIs.

In an , Monica Gandhi, MD, of the University of California, San Francisco, and colleagues, cautioned clinicians that the results of this study "should not be interpreted in isolation."

"Concerns about behavioral change after starting PrEP should not decrease the willingness of clinicians to offer PrEP," they wrote. "To continue to achieve the population-level influence on HIV incidence through PrEP that appears imminently achievable, PrEP will need to be available for and used by the populations that can benefit the most, including those having condomless sex."

Gandhi and colleagues commented on a , which found that "even if there was an increased rate of condomless sex among [men who have sex with men] using PrEP, the benefits of increased detection and early treatment of STIs could still eventually decrease the overall STI burden."

They suggested that the upcoming addition of PrEP as a preventive strategy for HIV infection recommended by the U.S. Preventive Services Task Force "could help pave the way for improved coverage of STI screening in the United States."

Limitations to the current data included the fact that gay and bisexual men who enroll in PrEP studies may not be indicative of gay and bisexual men using PrEP outside of a study environment. They also cited selection bias, and social desirability bias, as behavioral responses relied on self-reported data, the authors said.