Starting PrEP does not lead to more STIs, youth survey in Chicago finds

Ethan Morgan of Chicago's Centre for AIDS Research at IAS 2019. Photo: Gus Cairns

A survey of young gay and bisexual men and transgender women in Chicago, a proportion of them taking pre-exposure prophylaxis (PrEP), has found no direct relationship between starting PrEP and any increase in subsequent sexually transmitted infections (STIs). It did find that, not unexpectedly, people started having more condomless anal sex after starting PrEP, but this did not translate into more STIs.

Ethan Morgan of Chicago’s Northwestern University and Center for AIDS Research told the 10th International AIDS Society Conference in HIV Science (IAS 2019) is Mexico City that data were collected as part of RADAR, a cohort study of young men who have sex with men and transgender women aged between 16 and 29. 

A total of 744 young people who attended six or more study visits were followed out of a total cohort of 1100. During the study period, a third of participants started PrEP (32.9%). (The fact that this was only 21% in the pre-conference abstract submitted a few months ago shows evidence of a rapid expansion in PrEP use.) About a third were also diagnosed with a rectal STI: 17% with rectal gonorrhoea and 23% with rectal chlamydia (some with both).



The last part of the large intestine just above the anus.


Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 

adjusted odds ratio (AOR)

Comparing one group with another, expresses differences in the odds of something happening. An odds ratio above 1 means something is more likely to happen in the group of interest; an odds ratio below 1 means it is less likely to happen. Similar to ‘relative risk’. 

statistical significance

Statistical tests are used to judge whether the results of a study could be due to chance and would not be confirmed if the study was repeated. If result is probably not due to chance, the results are ‘statistically significant’. 


An umbrella term for people whose gender identity and/or gender expression differs from the sex they were assigned at birth.

The study looked longitudinally at participants’ PrEP starts and STI diagnoses and asked the question: if participants started PrEP at one study visit, was it associated with a higher rate of STI diagnoses in the next two study visits? The answer was no.

Although there was no statistically significant association either way, the highest rectal STI diagnosis rate was in fact observed in people before they stated PrEP.

The rates of any rectal STI in the three visits prior to stating PrEP, in the third of people who did, varied from 16 to 25% depending on which study period they were diagnosed in. In contrast the rates of rectal STIs in the same study visits in people who had already started PrEP ranged for 9 to 16%. These were similar to STI rates in people who never started PrEP (11 to 13%) and in people who used PrEP intermittently or irregularly (15% for any study period).

The overall adjusted risk ratio of PrEP use and STIs was 1.07, i.e. there were overall 7% more rectal STIs subsequent to being on PrEP, but this was not at all statistically significant and likely due to chance (95% confidence interval 0.63-1.82).

PrEP use was significantly associated with an increased likelihood of people reporting condomless anal sex at the next study visit (adjusted odds ratio (aOR) 1.61, i.e. 60% more condomless sex after starting PrEP), but condomless sex was not associated with subsequent STI diagnoses (aOR 0.95).

These findings held for all categories of participants and in particular were not associated with their race or ethnicity.

Subsequent studies will look at why more condomless anal sex did not lead to more STIs and whether this was due to higher testing and diagnosis rates or to a reduction in other sexual risks.


Morgan E et al. PrEP Use and STIs are not Associated Longitudinally in a Cohort Study of YMSM/TGW in Chicago. 10th IAS Conference on HIV Science, Mexico City, oral poster abstract no TUPDC0105, 2019. 

View the abstract on the conference website.

Download the study slides on the conference website.