Computer model says vaccines and treatment both needed to turn tide of HIV

This article is more than 17 years old.

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.

Glossary

mathematical models

A range of complex mathematical techniques which aim to simulate a sequence of likely future events, in order to estimate the impact of a health intervention or the spread of an infection.

microbicide

A product (such as a gel or cream) that is being tested in HIV prevention research. It could be applied topically to genital surfaces to prevent or reduce the transmission of HIV during sexual intercourse. Microbicides might also take other forms, including films, suppositories, and slow-releasing sponges or vaginal rings.

safer sex

Sex in which the risk of HIV and STI transmission is reduced or is minimal. Describing this as ‘safer’ rather than ‘safe’ sex reflects the fact that some safer sex practices do not completely eliminate transmission risks. In the past, ‘safer sex’ primarily referred to the use of condoms during penetrative sex, as well as being sexual in non-penetrative ways. Modern definitions should also include the use of PrEP and the HIV-positive partner having an undetectable viral load. However, some people do continue to use the term as a synonym for condom use.

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."

Further information on aidsmap

What a vaccine can do

References

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.