No intervention is going to be adopted by 100% of the men
eligible for it, of course. The researchers then modelled a range of scenarios
in which the coverage of each intervention – the proportion of eligible men
adopting it – was progressively reduced to 25%. This had a large effect on most
interventions, but not as large for some as one might expect. If only a quarter of gay men halved their number of repeated sexual partners, it would cut
the number of infections prevented by 75%; but infections were only reduced by 55% if only a quarter of gay men tested once a year and by 37% if only 25% of gay men tested twice a
Although immediate treatment had a relatively small effect by itself,
it had a larger effect when combined with other interventions, and a synergistic
effect if combined with regular testing. It was also the least affected by decreases
in coverage or increases in risk behaviour; this is due to the fact that, as it
reduces infectiousness anyway, such increases had less effect on HIV
transmissions. At 25% coverage of both, ‘test-and-treat’ had the same effect as
an individual intervention as PrEP would do.
In a probably achievable scenario a combination of PrEP in 25% of 'high-risk' men, 25% uptake of annual HIV testing in the 75% who did not use
PrEP, and 25% more men diagnosed with HIV put on immediate treatment reduced
the number of infections acquired up to 2020 by 44%.
If risk behaviours increase, then the reductions in HIV
infections will be much smaller, the model finds, but will not disappear
entirely. For instance if condom use with repeat sexual partners fell to zero, then combining PrEP (in 25% of HIV-negative
men) and annual testing (in the other 75%) would only result in a 12% decrease
in infections to 2020. On the other hand, if, in the same scenario, men doubled
the number of repeat sexual partners they had instead of dropping condoms, then
there would still be a 28% drop in HIV infections; this is because,
paradoxically, more repeat sexual partners would result in increased serosorting
behaviour in the 75% of men who were not on PrEP. However it would take at
least a 75% increase in most ‘risk compensation’ behaviours – including a
halving of the annual HIV testing rate – to produce a situation in which HIV
infections did not decrease. In other words the adoption of nearly any combination of
the interventions or behaviour changes proposed would be better than what we have
at the moment.
The provision of PrEP within a combination strategy as above
– especially if combined with increased HIV testing in those not on PrEP –
would, in a likely scenario, prevent more than 7000 of a projected 17,000 infections
by the end of the decade.
All models stand or fall by their inputs. For instance, if
PrEP adherence was very low, it would decrease the effect of this intervention.
However the effect of PrEP is actually likely to be greater than that predicted in this model: this is because the researchers
took 44% as their base-case effectiveness rate of PrEP, from the iPrEx study. But, the 86% effectiveness found in the PROUD study would considerably increase
the contribution of PrEP to reducing infection. However the researchers also found
that increasing the number of gay men on PrEP had a greater impact on new infections than an
increase in its effectiveness.