The Aids virus, HIV, has a relatively short life cycle of less than a few days from when it first infects a cell to the moment it completes the assembly of its "offspring" viruses. It also lacks any mechanism for correcting mistakes or mutations that occur when it converts its RNA genetic material into the DNA code of the human cell.
Taken together, these two factors mean that HIV possesses a formidable ability to produce new mutants that can evade any drugs designed to interfere with its short and uncomplicated life cycle. Drug resistance to any single anti-retroviral drug can, however, be overcome with stunning success by combining two or more anti-retrovirals – a double- or triple-whammy.
Combination therapy has been one of the great success stories of HIV treatment, and has allowed many thousands of infected people to live reasonably healthy and long lives. A suite of anti-retrovirals is now available that can attack HIV at different points in its life cycle.
When three or four anti-retrovirals are combined as part of a treatment regime known as highly active anti-retroviral therapy, the results are even more dramatic, as measured by the fall in the amount of free virus (called viral load) that circulates in the bloodstream.
It is this immense drop in viral load that makes anti-retroviral therapy so interesting as a possible weapon in fighting the wider epidemic. In the past, someone with HIV would typically be likely to pass on the virus to perhaps one person a year over the course of say 10 years.
Now, with a daily regime of anti-retrovirals, it appears that this number can fall below the important value required to sustain an epidemic – called the reproductive number. In effect, this means that drugs rather than vaccines could be used as a way of eradicating new infections.
But there are problems. These drugs have to be taken several times a day for life, otherwise drug resistance can quickly develop. They are expensive, and they are not free from side-effects.
However, unless and until a vaccine can be produced – and that seems as far off now as when research began 25 years ago – it may be that anti-retroviral drugs are all that we have left. But they would need to be used rigorously for at least the next 40 years, a significant and long-term investment, and one that drug companies would no doubt welcome.
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