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STI rates in PrEP users very high, but evidence that PrEP increases them is inconclusive
Gus Cairns, 2017-02-22 12:00:00

A study of PrEP users presented at the Conference on Retroviruses and Opportunistic Infections (CROI 2017) last week, from CROI’s host city of Seattle, showed that PrEP users had very high rates of STI diagnosis – in the order of 20 times higher than among HIV-negative gay men in the general population.

There was also an increase in STI diagnoses from a time point a year before they sought PrEP to the date they started it. And the percentage of men reporting never using condoms for anal sex somewhat increased while they were on PrEP, though never exceeding ten per cent of all PrEP users.

The evidence that STIs increased further while people were on PrEP was, however, a lot more ambiguous. Chlamydia cases increased from starting PrEP to nine months after starting it. On the other hand, syphilis diagnoses fell over the same time period while gonorrhoea diagnoses stayed at the same rate. The only STI which increased during PREP was urethral gonorrhoea. but that was only seen in a small number of individuals.

In a symposium, Professor Matthew Golden, who runs King County’s STI and HIV programme, told the conference that some other studies such as the Kaiser Permanente PrEP programme in northern California showed that STI rates increased after PrEP was started. Others, such as the PROUD study, provided little evidence of this.

The problem in proving that PrEP has any causal relationship to STIs is that STIs among gay men were, in general, rising before well before PrEP, and also that PrEP usually involves regular testing for HIV and STIs. Since many STIs are asymptomatic and self-limiting, more tests will result in more diagnoses.