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Are STI Rates Higher Among Daily PrEP Users?

A study conducted in the Netherlands has found that the incidence of sexually transmitted infections (STIs) is higher among men who have sex with men (MSM) who take daily pre-exposure prophylaxis (PrEP) for HIV prevention, compared with those who use event-driven PrEP. Event-driven PrEP is taken during the period beginning 24 hours before sex through 24 hours after sex.

PrEP: daily versus event-driven use

The demonstration study, labeled AMPrEP, looked at the incidence of HIV, bacterial STIs, and sexual behaviors of PrEP users at the STI clinic of the Public Health Service of Amsterdam. It examined the effect of daily versus event-driven PrEP on HIV and STI incidence in the MSM and transgender populations. The study enlisted eligible adults who were over the age of 18 and were HIV negative.

Eligibilty criteria for partcipants
The population included people reporting one or more of these behaviors in the 6 months prior to the study:1,2

  • Anal sex with casual partners without the use of condoms
  • One or more bacterial STIs
  • Use of post-exposure prophylaxis after a sexual risk incident
  • An HIV-positive sexual partner with detectable viral load (viral load is the term used to describe the amount of HIV in the blood.)

What were researchers looking for?

Eligible study participants were offered the choice between using a daily PrEP or event-driven PrEP therapeutic regimens. Every 3 months, they received HIV and STI testing and questionnaires about their sexual behavior practices that they filled out at each check-in.1,2

Researchers looked at the actual numbers of infections and calculated the incidence rate of HIV, chlamydia, gonorrhea, and syphilis. They included STIs diagnosed between study appointments. They also looked at personal behaviors including the number of sexual acts and sexual partners.

367 people participated in the study: 365 MSM and 2 transgender women. 269 selected daily PrEP and 98 chose event-driven PrEP.1,2

PrEP use and STIs

According to the study, both daily and event-driven PrEP regimens are effective in preventing the transmission of HIV. Of note is the finding that STI incidence was 41 percent lower in the event-driven PrEP cohort than in the daily PrEP group.2

Some significant takeaways include:1,2

  • Two people in the daily PrEP group became HIV-positive
  • 253 participants were diagnosed with one or more STIs during the first 2 years
  • STI incidence did not change over time
  • More than 25 percent of STIs were diagnosed from tests done in between study visits
  • The number of anal sex acts with casual partners and without condoms increased
  • The number of partners and sex acts remained stable

Other interesting takeaways were that in the first year on PrEP, 53 percent of the participants were diagnosed with more than one sexually transmitted infection. During the second year of the study, the number decreased to 51 percent of the remaining participants.1,2

Routine HIV and STI testing

Individual sexual behaviors may be responsible for the higher incidence of STIs among men who have sex with men who take daily pre-exposure prophylaxis for HIV prevention as compared to those who use event-driven PrEP.1,2 Researchers suggest that although it is not clear if STI incidence increases after initiation of and during PrEP use, STI prevention interventions should be tailored to behavioral profiles.1

They recommend that employing routine HIV and STI testing is critical in the PrEP population. Determining testing frequency, education, and prevention interventions should include review and consideration of individual sexual behaviors.1,2

Limitations to the study

There were some limitations to the study. Not all PrEP eligible populations were well represented. There were on only two transgender participants.

In addition, the investigators were unable to calculate the incidence of sexually transmitted infections before participants began PrEP therapy. This meant they were not able to compare incidence before and after PrEP.

They also suggested that to better understand the effects of behaviors while using PrEP, data collection needs to be evaluated over a longer period of time.1,2

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