Uptake of PrEP may be limited because few gay men think they are at risk of HIV infection

Toronto study highlights disconnect between 'objective' and 'subjective' assessments of risk
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Three quarters of gay men seeking HIV testing at a Toronto sexual health clinic were assessed as being at elevated risk of infection, but only a minority agreed with that assessment and were willing to use pre-exposure prophylaxis (PrEP), according to a study presented to Controlling the HIV Epidemic with Antiretrovirals: From Consensus to Implementation, a conference held in London this week.

The study found that men who did rate themselves at increased risk of infection were more likely to be willing to use PrEP, but that men 'objectively' at increased risk were no more likely than others to be interested in using PrEP.

However, it remains possible that the 'objective' screening tool used was not finely tuned enough to identify the men who would benefit most from new methods of HIV prevention.


post-exposure prophylaxis (PEP)

A month-long course of antiretroviral medicines taken after exposure or possible exposure to HIV, to reduce the risk of acquiring HIV.

multivariate analysis

An extension of multivariable analysis that is used to model two or more outcomes at the same time.


Studies aim to give information that will be applicable to a large group of people (e.g. adults with diagnosed HIV in the UK). Because it is impractical to conduct a study with such a large group, only a sub-group (a sample) takes part in a study. This isn’t a problem as long as the characteristics of the sample are similar to those of the wider group (e.g. in terms of age, gender, CD4 count and years since diagnosis).


Receptive anal intercourse refers to the act of being penetrated during anal intercourse. The receptive partner is the ‘bottom’.


Insertive anal intercourse refers to the act of penetration during anal intercourse. The insertive partner is the ‘top’. 

Numerous studies have investigated awareness and acceptability of PrEP in various populations. For example, among HIV-negative gay men, a London survey found that half were likely or very likely to take PrEP. Whereas younger men, previous users of post-exposure prophylaxis (PEP) and recent sexual health clinic attendees were more likely to be interested in using PrEP, men’s actual sexual behaviour did not predict willingness to use it.

An Australian study found that the majority of gay men were cautious about using PrEP, with 28% willing to use it. In contrast to the London study, those interested in taking PrEP were more likely to have recently had unprotected anal sex with casual partners and to have had more than ten partners in the past six months. As in London, they were also more likely to have taken PEP.

The Australian researchers also asked respondents, ‘How likely do you think it is you will become HIV-positive?’ (a question that has been included in very few other studies). The vast majority thought themselves unlikely to acquire HIV, with just 3.5% saying that it was likely or very likely. However, those who did think it likely were almost five times more likely than others to be willing to use PrEP.

The new study from Canada further explores men’s perception of risk. This is likely to be a significant issue in the uptake of PrEP, which so far has been slow. Taking PrEP each day requires men to anticipate that they will need extra protection from HIV, but with condoms frequently presented as being 100% effective and with non-condom use highly stigmatised in certain contexts, acknowledging that risk may be challenging for some individuals.

The researchers think that people who will adhere to PrEP need more than willingness – they also need to perceive themselves at risk of infection

The investigators recruited 423 men who have sex with men (MSM) at a downtown Toronto sexual health clinic. Average age was 30 and most were well-educated.

Overall, acceptability of PrEP was high, with 49.7% saying that they would be willing to take it as a daily tablet.

However, the researchers judged that people who will really take and adhere to PrEP need more than willingness – they also need to perceive themselves at risk of infection.

When asked about this, 17.0% of men thought that there was ‘more than a little risk’ or ‘a lot of risk’ that they would acquire HIV. Amongst this group, willingness to use PrEP was high, with three-quarters saying that they were willing. Indeed, in multivariate analysis, willingness to use PrEP was associated with this ‘subjective’ assessment of risk.

But far more men who were willing to use PrEP did not consider themselves at increased risk of infection (156 men, 36.9% of the whole sample) than did see themselves at increased risk (54 men, 12.8%).

And the researchers were also interested in ‘objective’ assessments of risk. There are few screening tools which clinicians can use to identify those at increased risk of infection, but the Centers for Disease Prevention and Control (CDC) have developed one for men who have sex with men, known as HIRI-MSM, which the Canadian researchers used.

Based on analysis of the risk factors for incident HIV infection in two large cohorts of American gay men in the late 1990s (Project Explore and VAXGEN 004), HIRI-MSM asks seven questions about age, partner numbers, receptive and insertive anal sex, HIV-positive partners and drug use. Men scoring 10 or more on the tool should, according to the CDC, have a more in-depth assessment of their sexual behaviour – this will allow clinicians to make decisions about provision of PrEP and other HIV prevention interventions.

However, the use the Canadian researchers made of the tool is slightly different. Men scoring 10 or more were simply defined as ‘objectively’ having high risk for HIV and therefore as eligible for PrEP.

And most of the men in this cohort of sexual health clinic attendees were at high risk by this reckoning – 77.1% scored over 10.

Is the ‘objective’ measure of risk used in this study the most appropriate one?

But in the statistical analysis, men ‘objectively’ rated as at increased risk of infection were no more likely than other men to be willing to use PrEP (whereas 'subjective' risk had been associated with willingness).

So, whereas one-in-seven of the men consider themselves at increased risk of infection (‘subjective’ assessment), the researchers considered three-quarters to be so (‘objective’ assessment).

Looking only at one quarter of the sample, those men with the highest HIRI-MSM scores (the top quartile), just 26.2% rated themselves at increased risk of infection. Similarly, of the one-in-ten men with the highest scores (the top decile), 27.3% thought themselves at elevated risk.

And just 46 of the participants (10.9%) satisfied all three of the following conditions – (a) willing to use PrEP, (b) subjective higher HIV risk, and (c) objective higher HIV risk. “This disconnect between objective and perceived HIV risk may pose a challenge when assessing individuals for PrEP,” say the researchers.

More work is needed to develop appropriate ways of identifying individuals who would benefit from PrEP. Within this, it is worth asking:

  • Is the ‘subjective’ measure of risk used in this study the most appropriate one? It is challenging for individuals to describe themselves as ‘likely’ to become HIV positive, and it is not certain that people who do not do so are unwilling to take steps to avoid infection.
  • Is the ‘objective’ measure of risk the most appropriate? While HIRI-MSM is rigorously based on incidence data, the cut-off score of 10 may be too low to only identify those at highest risk of seroconversion. In fact, any man reporting receptive anal sex instantly scores 10 (regardless of condom use or partner numbers), while men aged 18 to 28 have 8 points added to their score. The tool may be best used to exclude individuals who don’t need further assessment (99.5% of those with a score below 10 did not acquire HIV in the next six months), rather than as a tool to identify those at the highest risk (1.9% of those with a score above 10 did acquire HIV in the next six months).

Kain T et al. Low Perceptions of HIV Risk among Toronto MSM Seeking Anonymous HIV Testing: Objective and Subjective Assessments of PrEP Eligibility. Abstract 22, Controlling the HIV Epidemic with Antiretrovirals, London, September 2013.

Smith DK et al. Development of a Clinical Screening Index Predictive of Incident HIV Infection Among Men Who Have Sex With Men in the United States. Journal of Acquired Immune Deficiency Syndromes 60: 421-427, 2012.