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Computer model says vaccines and
treatment both needed to turn tide of HIV
http://www.aidsmap.com/news/newsdisplay2.asp?newsId=2263
 22
August 2003
Julian Meldrum
A mathematical model of the HIV epidemic in the
Rakai District of Uganda has been used to explore the likely
impact of introducing antiretrovirals and/or a low-efficacy
vaccine. A paper in the journal AIDS by Johns Hopkins
University researcher Ronald Gray and the Rakai Project study
team projects the course of HIV and AIDS over 20 years in a
population where there is a well-studied, large-scale
heterosexual epidemic. Using a range of assumptions about the
uptake and efficacy of medical interventions, it tests the
idea that either treatment or partially effective vaccines
could turn the tide of HIV in the Rakai District.
The researchers find, firstly, that ARV treatment
on its own will not be enough to reduce the number of people
living with the virus, unless it goes well beyond current US
guidelines. Those guidelines would mean treating when viral
load is > 55,000 copies (which would cover 20% of the Rakai
HIV positive population) as well as when CD4 counts fall below
350. If treatment achieved a similar level of success to US
treatment, then the number of people with HIV in Rakai would
stabilise but not decline below current levels over 20 years.
In fact, as the authors observe, even this level of treatment
is beyond current WHO recommendations and would be hard to
implement in Rakai, where neither CD4 nor viral load tests are
currently provided. The alternative of universal treatment for
people with HIV would raise serious ethical problems, as it
shifts the balance of risks and benefits for treated
individuals and, of course, would be even harder to finance.
One implication, not spelled out, is that if
viral load tests can be made practical and affordable - for
example, through using dried blood spots - then there could be
public health benefits from offering treatment on that basis.
The mathematical model shows that a low- or
moderate-efficacy vaccine could turn the epidemic around far
more readily than treatment could, but its impact depends
heavily on the level of coverage achieved and is undermined if
there is a shift towards riskier sexual behaviour among
vaccinated people.
A factor missing from the model is the effect of
treatment on the uptake of HIV testing and the willingness of
positive people to disclose their status to family, friends
and sexual partners, and the effect of these, in turn, on
people's readiness to accept vaccination against HIV.
Combining treatment and a vaccine - even a low
efficacy vaccine - may have a less dramatic effect on the
proportion of the population living with HIV than providing a
vaccine alone, since the reduced death rates on treatment slow
that decline.
In reality, however, for the reasons stated in
the previous paragraph, vaccine uptake is likely to be higher
in a population that has greater access to treatment. Keeping
people with HIV alive for additional years to care for their
children and contribute to society has immense value in
reducing the harmful impact of the epidemic, and it is hard to
capture this in a mathematical model. This may easily outweigh
the 'cost' of having a higher proportion of the population
living with HIV.
Exactly the same arguments have been applied by
other researchers to the introduction of microbicides and
other interventions to prevent HIV and AIDS. And, as the paper
stresses, the need for established prevention methods is not
diminished: "the promotion of safe sex is critical to the
public health benefits that might accrue from ART or
vaccines".
The authors conclude: "improved access to
ART is needed for eligible HIV-infected persons in developing
and developed countries to improve survival and quality of
life. However, there is also a need to consider the societal
benefits of therapy in terms of control of the HIV epidemic.
ART initiated only for persons with advanced disease is
unlikely to reduce HIV transmission sufficiently to control
the epidemic." ... "A preventive vaccine, were it to
become available, offers the best hope of controlling the
epidemic in the long-term, particularly if there was a
concurrent ART program, or if the vaccine itself could reduce
viral load in HIV-infected persons."
Reference
Gray RH et al. Stochastic simulation of the
impact of antiretroviral therapy and HIV vaccines on HIV
transmission, Rakai, Uganda. AIDS 17:1941-1951, 2003.
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