Background: To meet individual HIV prevention needs for people whose risk may vary over time, daily (one tablet emtricitabine-tenofovir daily) and event-driven (2 tablets before, and 2 times 1 tablet after sex) pre-exposure prophylaxis (dPrEP and edPrEP, respectively) can be offered at choice of the user. We assessed user-initiated switches and factors associated with switches during the first two years of follow-up.
Methods: HIV-negative MSM (n=370) and TGP (n=2) participated in the Amsterdam PrEP project at the Public Health Service of Amsterdam. Participants chose a dPrEP or edPrEP regimen and could switch regimen at quarterly visits. We analyzed data from study start (August 2015) through March 2018. We evaluated time-to-first-switch overall and by PrEP regimen using Kaplan-Meier plots. With Cox regression we assessed the association of baseline characteristics with time to switch, in separate models for dPrEP and edPrEP.
Results: At baseline, 270 (73%) people chose dPrEP and 102 (27%) edPrEP. Medium follow-up time was 2 years [IQR 1.8-2.2]. In total, 106 (28%) participants switched regimen 154 times; 37/106 (35%) switched more than once. 78 (51%) switches were from dPrEP to edPrEP, and 76 (49%) from edPrEP to dPrEP. The cumulative proportion of participants that switched two years after PrEP start was 30% (95% CI 25-35%). Over time, the cumulative proportion that switched was higher among those who started edPrEP compared to those who started dPrEP (log rank test, p< 0.001) (Fig).
Higher age was associated with switching (Table) for both PrEP regimens. Among those who started edPrEP, a higher number of sex acts with casual partners (reported at baseline) was associated with a higher switch hazard. Other baseline variables were not significantly associated with the switch hazard, including ethnicity, STI at baseline, and chemsex (Table).
Conclusions: A substantial proportion of MSM switch between daily and event-driven PrEP regimens if allowed to do so. Some participants may have switched without reporting, suggesting possible underreporting. The high number of switches, in addition to 27% initiating event-driven PrEP, clearly indicates the varying needs and added value of a client-centered approach in PrEP regimen provision.

Time to first switch to another PrEP regimen since start PrEP, (a) overall and (b) by PrEP regimen
[Time to first switch to another PrEP regimen since start PrEP, (a) overall and (b) by PrEP regimen]

 Switch dPrEP --> edPrEP Hazard ratio (a) (95% CI)Switch edPrEP --> dPrEP Hazard ratio (a) (95% CI)
Age in years at baseline0.30 (0.11-0.77) (b)0.24 (0.07-0.81) (b)
Ethnicity (white)0.96 (0.47-1.97)1.74 (0.57-5.37)
No. of anal sex partners in 3 months before PrEP start0.71 (0.37-1.36) (b)0.54 (0.27-1.09) (b)
No. of anal sex acts with casual partners in 3 months before PrEP start1.20 (0.66-2.19) (b)2.31 (1.14-4.67) (b)
Chemsex in 3 months before baseline1.10 (0.59-2.05)0.82 (0.37-1.82)
Any STI at baseline0.53 (0.23-1.19)1.59 (0.65-3.88)
Possible drug-related problem (DUDIT≥8) at baseline1.08 (0.56-2.07)1.65 (0.73-3.71)
Possible alcohol-related problem (AUDIT≥8) at baseline 1.06 (0.59-1.89)0.72 (0.33-1.57)
(a) Adjusted for all other variables in the model; (b) Per increase in natural log. Abbreviations: AUDIT, alcohol use disorders identification test; DUDIT, drug use disorders identification test; IQR, interquartile range; PrEP, pre-exposure prophylaxis; STI, sexually transmitted infection
[Association of baseline characteristics with time to switch, by initial PrEP regimen.]