As PrEP is recommended to populations at a higher risk for acquiring HIV, some clinicians have grown concerned that use of PrEP could be associated with an increased incidence of bacterial sexually transmitted infections (STIs).

Now, a new study by a team of Australian investigators published in the Journal of the American Medical Association reports that, among gay and bisexual men, receipt of PrEP during the study was associated with an increased incidence of STIs when compared with before the study period.

The team set out to describe STI incidence as well as behavioral risk factors among gay and bisexual men using PrEP, in addition to exploring changes in STI incidence following initiation of a PrEP regimen.

The research was conducted as part of the Pre-exposure Prophylaxis Expanded (PrEPX) study, which was a multisite, open-label intervention study within the Australian Collaboration for Coordinated Enhanced Sentinel Surveillance (ACCESS) clinic network. The study enrolled a total of 4275 participants between July 26, 2017, and April 1, 2018, in Victoria, Australia.

Of the 4275 total participants in PrEPX, 2981 were enrolled into this study across 5 clinics—3 of which were primary care, 1 sexual health, and 1 community-based HIV rapid testing service. Upon enrollment, the participants received daily oral tenofovir disoproxil fumarate and emtricitabine for HIV PrEP, along with quarterly HIV and STI testing and clinical monitoring. Each participant and had at least 1 follow-up visit and was monitored until April 30, 2018.

The median age of the participants was 34 years (interquartile range 28-42) and 98.5% of the study population identified as gay or bisexual males. Additionally, 29% of participants reported used PrEP prior to enrollment and 89 (3%) withdrew and were censored at date of withdrawal; therefore, 2892 (97%) were included at final follow-up.

The primary outcome of the study was incidence of chlamydia, gonorrhea, or syphilis. The investigators note that incidence rates and hazard ratios describing behavior risk factors of STI diagnosis were calculated and that incidence rate ratios (IRRs) were adjusted for change in testing frequency, and described changes in STI incidence from 1-year prior to study enrollment to study follow-up among 1378 participants with pre-enrollment testing data.

The investigators found that during a mean follow-up of 1.1 years (3185 person-years), there were 2928 STIs diagnosed (1434 chlamydia, 1242 gonorrhea, 252 syphilis) among 1427 (48%) of the total participants. Overall, the STI incidence was 91.9 per 100-person-years, with 736 participants (25%) accounting for 2237 (76%) of all STIs.

According to multivariable analysis, although younger age, greater number of sexual partners, and group sex were associated with greater STI risk, condom use was not.

Based on pre-enrollment testing data that was available for 1378 participants, the incidence of STIs increased from 69.5 per 100 person-years prior to the study to 98.4 per 100 person-years during follow up (IRR 1.41 [95% CI, 1.29-1.56]).

Following the adjustment for testing frequency, the increase in incidence from 1 year prior to the study until follow-up was reported as “significant for any STI (adjusted IRR, 1.12, CI, 1.02-1.23) and for chlamydia (adjusted IRR, 1.17, CI, 1.04-1.33).”

Based on these findings, the investigators concluded that among gay and bisexual men using PrEP, STIs were highly concentrated among a subset of study participants. Additionally, receipt of PrEP at study enrollment was associated with an increased incidence of STIs compared with pre-enrollment.

Overall, these findings place emphasis on the importance of frequent STI testing among gay and bisexual men using PrEP.

The investigators concluded that the study results “suggested that STI prevention campaigns should not focus solely on condom use but also on reducing the time to STI diagnosis and treatment by promoting easy access to frequent testing.”