Australia had its first pilot studies of PrEP in 2014, gave regulatory approval in early 2016 and began to reimburse most of its cost in March 2018. Uptake has been rapid in Melbourne (the capital of the state of Victoria) – 6% of gay men were using PrEP in January 2015, rising to 18% in January 2018.
Many of those taking PrEP are participants in PrEPX, an implementation study at ten clinics in a range of locations. The study’s primary objective is to see if the use of PrEP is associated with a reduction in HIV infections across the state, but the conference presentation gave interim findings on the study’s secondary outcome – sexually transmitted infections.
Data come from 2981 of the study’s 4275 participants, those who enrolled at a clinic that contributes enhanced surveillance data as part of the ACCESS collaboration. Almost all (98%) are gay and bisexual men, their median age is 34 years, 48% had had condomless receptive anal sex in the three months before enrolling and 28% had used PrEP before.
During follow-up of a little more than a year, 52% had no STIs. STIs were highly concentrated in a minority of PrEP users who had repeat infections – 25% of participants had two or more infections, accounting for 76% of infections. Thirteen per cent of participants had three or more infections, accounting for 53% of infections.
The overall incidence of STIs, averaged out across all participants, was 91.9 per 100 person-years. This means that in a group of 100 people followed for one year, there would be around 92 STI diagnoses. It does not mean that 92 of the 100 men would have an STI – a few men would have multiple STIs during the course of the year.
The most common infections were chlamydia and gonorrhoea. Incidence of rectal infections was 56.6 per 100 person-years; urethral infections 22.4 per 100 person-years; and pharyngeal infections 23.5 per 100 person-years.
The next analysis looked at 1378 participants who had been attending the same clinic before they enrolled in PrEPX. This allowed the researchers to compare STI incidence in the 12 months before enrolment with STI incidence while in PrEPX.
Among men who had taken PrEP before joining PrEPX, incidence was 92.4 per 100 person-years before joining the study and 104.1 per 100 person-years while in PrEPX. This difference was not statistically significant.
However, incidence was greatly increased for men who took PrEP for the first time – from 55.1 per 100 person-years to 94.2 per 100 person-years (a 71% increase, p = < 0.001). An important factor that needs to be taken into account when looking at any data on this issue is that PrEP users are required to test for STIs every three months, thus multiplying the opportunities for infections to be diagnosed. The testing rate increased by 48% in people taking PrEP for the first time, which introduces a ‘detection bias’.
After making statistical adjustment for this, STI incidence increased by 21%. This remained statistically significant (p = 0.006) but is of moderate clinical significance.
The main behavioural factors which were associated with STIs were having more sexual partners and having group sex more frequently. Using condoms more or less often did not make any difference to STI rates. This suggests that interventions to reduce repeat STIs in PrEP users should focus more on partner numbers and on group sex than on condom use.