Sexually transmitted infection testing and prevalence among MSM using event-based dosing HIV pre-exposure prophylaxis

Keywords: Chlamydia trachomatis, HIV prevention, Neisseria gonorrhoeae, pre-exposure prophylaxis, sexual behaviour, sexually transmitted infections, sexually transmitted infection testing, Treponema pallidum.

Event-based dosing (EBD) of HIV PrEP has been shown to be as effective at preventing HIV transmission in MSM as daily dosing.^1,2 The use of HIV PrEP in MSM is associated with incident sexually transmitted infections (STI).³ There are limited data on the differences in STI testing frequency between MSM using EBD and daily dosing PrEP.^4–6 Brighton, United Kingdom, has a large MSM population served by a single sexual health clinic and we offer both EBD and daily PrEP. We aimed to examine any differences in demographics, sexual partners, STI testing, STI incidence and recreational drug use between daily PrEP and EBD PrEP users.

We performed a cross-sectional analysis of PrEP-users between January 2020 and January 2021. We collected anonymised data on demographics (age, gender identity, sexuality and ethnicity), smoking status, reported number of sexual partners in the previous 3 months, number of STI tests, use of post-exposure prophylaxis, diagnosed STIs (chlamydia, gonorrhoea, early syphilis) and hepatitis C risk factors (chem-sex, group sex, fisting). We used Fisher’s exact and Kruskal–Wallis tests to compare the EBD and daily users.

There were 418 individuals who attended for PrEP during this time period: 402 (96%) MSM, 1 (0.2%) cis-woman and 9 (2.1%) transgender people. The median age was 41 years old (IQR = 32–50) and 300 (72%) described their ethnicity as white. Overall, 83 (20%, 95% CI = 16.1–24.0) were using EBD PrEP. There were no differences in: post-exposure prophylaxis following sexual exposure use in the last 12 months (0 in EBD-PrEP vs 1% in daily dosing, P = 1); hepatitis C risk-taking behaviour (16% vs 12%, P = 0.35); injecting drug use in the last 12 months (2% vs 0.9%, P = 0.26); chem-sex in the last 12 months (8% vs 4%, P = 0.07); group sex in the last 12 months (7% vs 8%, P = 1), fisting in the last 12 months (2% vs 2%, P = 0.63), reported non-consensual sex (1% vs 2%, P = 1) or current smokers (13% vs 19%, P = 0.26). EBD-PrEP users were less likely to: have >5 sexual partners per year [22 (27%) vs (142 (42%), P = 0.008] and have >4 STI tests per year [15 (18%) vs (169 (50%), P < 0.001]. There were no differences in STI rates (chlamydia, gonorrhoea, or syphilis) between EBD-PrEP and daily users (35% vs 34%, P = 0.54), (Table 1).

Table 1.  Daily vs event-based PrEP users (N = 418).

We have shown that 20% of PrEP users in our study were using EBD and these individuals had a lower number of sexual partners and tested for STIs less frequently than daily PrEP users but were diagnosed with a similar number of STIs. The proportion of MSM using EBD-PrEP in this study is similar to other high-income settings but is in contrast with studies from Asia.^4–8 Globally, many factors influence the choice of PrEP regimen, including PrEP awareness among MSM and clinicians, drug and alcohol use and privacy issues including being married to or living with a cis-female.^9–11 Furthermore, there are financial implications for individuals and public health on the choice of PrEP regimen, including the optimal STI testing frequency; avoiding unnecessarily frequent tests which may increase the uptake of EBD-PrEP. Modelling data in Europe suggests that reducing STI testing frequency in asymptomatic MSM using PrEP, can result in significant delays in STI diagnosis including syphilis.¹² The increased testing for STIs in MSM driven by PrEP use is associated with increased macrolide use and risk of increasing rates of antimicrobial resistance.¹³ A delayed diagnoses of syphilis due to a reduction in STI screening in MSM using PrEP could result in serious sequelae. There are several limitations to our study including being a single centre clinic-based study which included patients attending for PrEP during the COVID-19 pandemic which may have affected access to services.

More research is needed to understand the optimal frequency for STI testing in MSM using EBD-PrEP, and until then, we need to increase opportunities for STI testing for MSM using EBD-PrEP.

All the data from this work is in the manuscript.

None declared.

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

DR, KS and CF designed this study, CF performed the data collection. KS analysed the data. KS, CF and DR all contributed to the final manuscript.