A number of studies have attempted to quantify the
possible impact of PrEP under a number of different efficacy, usage and
behavioural scenarios. A
2007 study presented in PLoS One journal found that PrEP could prevent
up to three million new HIV infections over ten years in southern Africa if used consistently, according to mathematical
set out to model the effects of PrEP under different scenarios in Africa, varying the efficacy of PrEP and the effects of
sexual disinhibition within the population after its introduction in order to
illustrate the range of effects that might be expected.
model looked at the effect of introducing PrEP in epidemics where adult HIV
prevalence has reached 20%.
were three scenarios within the model:
- An optimistic
scenario in which PrEP was assumed to be effective 90% of the time and used by
75% of the sexually active population. In this case, there was a significant
public health benefit with a reduction of new HIV infections by 74%.
- A neutral scenario
in which PrEP was effective 60% of the time and used by 50% of the sexually
active population. This gave a 25% reduction in infections.
- A pessimistic
scenario in which PrEP was effective 30% of the time and used by 25% of the
sexually active population. A 3.3% reduction was found.
researchers also looked at the cost-benefit of distributing PrEP and found that
targeting it to individuals who were the most sexually active produced a
significant decline in infections at a much lower cost than if PrEP were
distributed to the general population.
address the issue of sexual disinhibition, the researchers assumed a 100%
increase in risky sexual behaviour and observed that there was still a notable
reduction of HIV infections in the range of 23.4% to 62.7%.
2010 Microbicides conference, a couple of presentations modelled the impact of
either a microbicide or PrEP, and Marie-Claude Boily of Imperial College
introduced a meta-review of a number of the mathematical models of these
models show that the effect on HIV incidence and prevalence of microbicides and
PrEP could vary enormously, according to local conditions. In some cases their
adoption could cause an increase in HIV infections. Boily said that models
predict some paradoxical effects.
might have much more effect in a low-prevalence area than a high-prevalence
one, and it might reduce HIV infections in people who don’t often use condoms,
while considerably increasing infections in people who were using them
consistently before the introduction of PrEP.
PrEP or microbicide adoption were to result in anything more than a slight drop
in condom use, the result could be an increase in HIV cases; and even a small
amount of anal sex in a heterosexual population may slash the effectiveness of
a vaginal microbicide.
presented several different scenarios that modelled condom substitution, all
assuming the introduction of a prevention method of 50% efficacy, used 50% of
of initially high condom use were much more sensitive to condom substitution:
if condom use was initially 90%, it would only take a 2% drop in use for
infections to start rising, despite the introduction of a microbicide. Audience
members commented that this validated the fears of groups representing female sex
workers, who have managed to enforce this level of condom use in clients. If
microbicide use led men to demand more sex without condoms, then both worker
and client would be more at risk.