BBC News - Tuesday, 6 November, 2001
The finding - detailed in a leading US medical journal on Tuesday - makes worrying reading for doctors who use drug therapy to keep the virus under control.
Modern antiretrovirals can be remarkably successful against HIV, keeping patients alive and well for many years, despite not actually curing the infection.
However, HIV is constantly changing its genetic makeup and finding forms which the drugs are not as effective against.
The latest type of the virus - detected in large numbers of patients in the US - has a genetic make-up which allows it not only to replicate easily, but to mutate rapidly as well.
This capacity for mutation is likely to boost the virus' ability to resist AZT - the oldest, and still most important anti-HIV drug.
New concerns
Researchers at the Division of Aids at the National Center for Infectious Diseases in Atlanta, Georgia, said their discovery "raised concerns" about the potential of the virus to "compromise the efficacy" of antiretroviral therapy.
Any resistance to AZT is important, even though there are well over a dozen different anti-retroviral drugs.
This is because anti-HIV drugs are normally given in combinations of two or more at a time.
AZT figures in a large number of these established combinations - if it lost its potency against a large number of HIV strains, then doctors would have lost a large part of their armoury against the infection.
HIV charities are worried that people will over-estimate the effectiveness of anti-HIV drugs and stop taking as many precautions to protect themselves during sex.
Lisa Power, from the Terrence Higgins Trust, said: "This research clearly shows why we cannot afford to let up on our efforts to halt the spread of HIV.
Safer sex
"Combination drug therapies, which have been instrumental in enabling people with HIV to lead full lives, are powerless against emerging untreatable strains of the virus.
"Safer sex is still the best defence against HIV."
Multidrug resistance - where HIV strains have become resistant to more than one drug - are the biggest fear of doctors, and at least one case has already emerged in Canada.
"If we get multidrug resistance, that could put us back to where we started when HIV first emerged."
There is research into new classes of anti-HIV drugs, but because of the efficacy in recent years of conventional therapies, it is proving much more difficult to recruit volunteers to take part.
The research was published in the Proceedings of the National Academy of Sciences.
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