Attendees at the 18th European AIDS Conference (EACS 2021), held in London, today learned the first efficacy data from what is likely the largest PrEP demonstration study ever conducted: the IMPACT PrEP Implementation Trial in England. Gay and bisexual men enrolled in IMPACT had 87% fewer HIV infections than a comparable group of sexual health clinic attendees who did not take PrEP.
IMPACT may be the largest demonstration study of PrEP against HIV ever conducted, with 24,255 participants. Nearly 96% were cisgender GBMSM men, and this analysis has focused on them.
The results of these individuals were compared with those of other cisgender GBMSM men who attended sexual health clinics and who, although they did not take PrEP, would have been eligible to do so given their risky sexual behaviors.
During the course of the study, 24 participants were diagnosed with HIV. In all but one case, the cause of infection was likely related to poor adherence to or discontinuation of PrEP. The annual incidence rate was 0.13%, compared to 1.01% in the comparison group of men not using PrEP.
The research team attempted to use routinely collected data (on sexually transmitted infections (STIs), post-exposure prophylaxis, HIV testing, sex work, and known HIV-positive partners) to predict the risk of acquiring HIV. However, it turned out that among men without these risk factors, the incidence was higher than among men with higher-risk markers. This was true regardless of whether or not they were taking PrEP. This suggests that these markers cannot currently be used to identify which individuals seen at sexual health clinics are most likely to benefit from PrEP.
Regarding STIs, high rates were recorded, concentrated in a minority of the study participants. Thus, while half of the men had no STIs, 26% of the participants were responsible for 80% of the STIs diagnosed in the study.