Effect of Daily vs Event-Driven PrEP on HIV and STI Incidence in MSM

Man holding a pill.
Man holding a pill.
The incidence of STIs was higher among men who are HIV negative and have sex with men using daily PrEP compared with those using event-driven PrEP.

The incidence of sexually transmitted infections (STIs) was higher among men who are HIV negative and have sex with men (MSM) using daily pre-exposure prophylaxis (PrEP) compared with those using event-driven PrEP, but the overall incidence of STIs did not change during two years of PrEP use. This is according to data published in The Lancet HIV.

During August 2015 and May 2016, 367 MSM and who were HIV negative aged ≥18 years and transgender people who reported ≥1 of the following in the previous 6 months: condomless anal sex with casual partners, at least 1 bacterial STI, use of postexposure prophylaxis after a sexual risk incident, or an HIV-positive sexual partner with detectable viral load were recruited to the study. Participants were given a choice of either daily PrEP or event-driven PrEP regimens. and were seen every 3 months for HIV and STI testing and to gather results of self-administered questionnaires about sexual behavior.

A total of 681.7 person-years of follow-up was contributed by the 365 MSM and 2 transgender women who participated. Of the participants, 269 chose daily PrEP and 98 chose event-driven PrEP. Within the daily group, 2 individuals became HIV positive and overall the HIV incidence rate was 0.30 per 100 person years (95% CI, 0.07–1.19). In the first year on PrEP, 193 (53%) of 367 participants were diagnosed with ≥1 STIs (incidence rate 93.5/100 person-years). During the second year on PrEP, this decreased to 175 (51%) of 343 participants (incidence rate 86.9/100 person-years). This corresponded to an overall incidence rate of 90.4 per 100 person-years

STI incidence did not change over time (adjusted incidence rate ratio [aIRR] 1.01 per 3-month follow-up; 95% CI, 0.98-1.05; P =.55). More than one-quarter of all STIs were diagnosed from tests done between study visits. The incidence of STI in the event-driven PrEP group was 41% lower than in the daily group (aIRR 0.59; 95% CI, 0.46–0.75). The numbers of partners and sex acts remained stable but the number of condomless anal sex acts with casual partners per 3 months increased.

Investigators could not calculate STI incidence before PrEP initiation and therefore could not compare before and after incidence. Investigators also noted that to adequately study the effect of PrEP on behavioral changes, data collection for a longer period before initiation of PrEP would be required. In addition, despite efforts made by investigators to reach all PrEP eligible populations, few transgender people were included, making the results difficult to generalize to this group.

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The data did show that STI incidence was stable over time in users of both daily and event-driven PrEP and that condom use decreased mostly among daily PrEP users. The STI incidence was higher among daily PrEP users as well. According to investigators, because more than a quarter of STIs were diagnosed between visits, “studies should strive to include all STIs diagnosed in their analysis, with harmonization of STI data collection across studies.” Researchers also concluded that both PrEP regimens are effective for the prevention of HIV but that regular HIV and STI testing remains of great importance among MSM using PrEP. Investigators suggested that, “frequency of STI testing, and behavioral and other STI prevention interventions might be tailored specific to sexual behavior,” and these strategies need to be considered by clinicians and policy makers.


Hoornenborg E, Coyer L, Achterbergh R, et al. Sexual behaviour and incidence of HIV and sexually transmitted infections among men who have sex with men using daily and event-driven pre-exposure prophylaxis in AMPrEP: 2 year results from a demonstration study. Lancet HIV. 2019;6:PE447-E455.