No evidence of risk compensation in female sex workers starting PrEP

Researchers found no evidence that female sex workers in Benin engaged in higher-risk behaviours after starting pre-exposure prophylaxis (PrEP). A novel methodology was used in order to measure this: the presence of biomarkers indicating exposure to semen in vaginal samples.

The researchers found that the percentage of women testing positive for semen exposure did not significantly change after the introduction of PrEP. Prevalence of sexually transmitted infections (STIs) in the sample decreased from 16% to 2% over a period of 24 months, suggesting that PrEP engagement provided an opportunity to monitor and control STIs. This research was carried out by Katia Giguère of the Université Laval in Canada and colleagues and was published ahead of print in the Journal of Acquired Deficiency Syndromes.

While PrEP is an effective means of preventing HIV infection, its use has raised concerns regarding risk compensation: an increase in behaviour such as condomless sex based on the assumed protection gained from using PrEP. This also raises concerns regarding whether PrEP users are more at risk for contracting STIs due to increased risk-taking behaviour. Results from research have been mixed: one review of PrEP use in different populations found no association between PrEP use and an increase in risky behaviour (as measured by incidence of STIs and self-report) while another review of PrEP use in men who have sex with men found evidence for an increase in condomless sex also measured by self-report and STI incidence. A PrEP study among female sex workers in South Africa showed a decrease in STIs and no clear change in self-reported condom use over time after starting PrEP.

Finding a clear association between PrEP use and risk-taking behaviour can be challenging. Self-reported data is often subject to social desirability bias. In these instances, research participants may not report on condom use accurately due to concerns about how they will be viewed by researchers. Reports of condom use may also change as a result of counselling and information received during the course of a study or due to repeated investigation of sexual behaviours. Additionally, using STIs as evidence of condom use also presents challenges: not all condomless sex results in STIs and a decrease in STIs may be as a result of timely treatment as opposed to condom use over time.

The study

Two hundred and fifty-five adult female sex workers in Benin were enrolled in a prospective PrEP observational study for a period of 24 months from 2014 to 2016, with follow-up every six months. In addition to self-reported instances of unprotected sex and measuring the incidence of STIs over this period, researchers also used novel biomarker measures as an indication of having unprotected sex due to some of the challenges with measuring risk-taking behaviour accurately.

Two measures of semen exposure in vaginal samples were used at baseline and every follow-up visit: prostate-specific antigen and Y-chromosomal DNA. After semen exposure, prostate-specific antigen and Y-chromosomal DNA can be detected in vaginal samples for up to two and 14 days respectively. These measures could then be compared to self-reported data of unprotected sex in order to see if under-reporting of condomless sex was occurring.

Average age of participants was 32.5 years, the majority (66%) had less than secondary level education and 98% were unmarried. Most women identified condoms as an effective means of preventing HIV (98%), viewed themselves as at-risk for contracting HIV (89%) and viewed the risk of HIV infection as low when taking PrEP (81%).

Results

"Biomarkers of exposure to semen did not change significantly over the course of the study, indicating no evidence of risk compensation."

At baseline, 16% of women tested positive for STIs (including gonorrhoea, chlamydia and trichomoniasis). While there were a high number of participants who did not complete follow-up to 24 months (151 completed follow-up to 12 months and only 30 of the participants completed follow-up to 24 months), there was a statistically significant decrease in STIs from 16% to 2%. However, it is important to note that this represents prevalence, not incidence (new infections) and is most likely attributable to the treatment received over the course of the study.

Biomarkers of exposure to semen did not change significantly over the course of the study, indicating no evidence of risk compensation. At baseline, a total of 32% of women tested positive for prostate-specific antigen (exposure to semen in the past two days) and 44% tested positive for Y-chromosomal DNA (exposure within the past 14 days). At 12 months, 34% tested positive for prostate-specific antigen while 33% tested positive at 24 months. In terms of Y-chromosomal DNA, 41% tested positive at 12 months and 47% at 24 months.

Researchers also wanted to see if participants were significantly under-reporting condomless sex, using biomarkers as evidence. They found that there were significant declines in self-reports of condomless sex over time, but no significant changes in the biomarkers of semen exposure.

Conclusion

“Our study has shown no evidence of risk compensation, and a decrease in STI prevalence following PrEP implementation,” conclude the authors. “Our results suggest that a PrEP intervention may be a great opportunity to provide sustained STI screening and treatment for a better control of STI epidemic among this population. Noticeably, our results also suggest that bias in self-report of unprotected sex may vary over the course of a longitudinal study. Those results are concerning and point out the necessity to objectively assess trends in unprotected sex by the means of biomarkers.”