When the drug was given to Aids patients it was able to block the progression of a serious eye infection often associated with the disease.

Scientists believe the only way the drug was able to do this was by boosting the immune systems of the Aids patients, making them more able to fight off other serious infection.

Researchers from the US National Institutes of Health (NIH) tested highly active antiretroviral therapy (HAART) on 14 Aids patients suffering from the eye infection cytomegalovirus (CMV) retinitis.

Writing in the Journal of the American Medical Association (JAMA), they report that all the patients were able to stop taking standard anti-CMV medications safely, and without progression of their CMV retinitis.

Dr Carl Kupfer is director of the National Eye Institute (NEI) which conducted the study in collaboration with the National Institute of Allergy and Infectious Diseases (NIAID).

He said: "CMV retinitis is not progressing in patients who are receiving HAART, suggesting that somehow HAART is playing a role in strengthening the immune system.

"With HAART, the rejuvenated immune system effectively controls this serious infection and doesn't need the help of specific anti-CMV retinitis medications.

"Stronger immune systems respond better to HIV and other infections, potentially allowing patients to live longer."

Combination of drugs

HAART is a combination of drugs taken orally. It usually includes at least one protease inhibitor and two other antiretroviral agents, but varies for each patient.

The combination is already known to boost the number of immune system T cells that fight disease.

Dr Kupfer said that HAART would change clinical practice and lead to a significant improvement in the quality of life for people with Aids who have CMV retinitis.

Standard anti-CMV treatments have to be taken for life, and can have serious side effects, including kidney damage and a reduction in blood cell counts.

In many patients they fail to stop progression of the disease.

Another indication of HAART's success was that throughout the trial HIV levels remained low or below detectable limits, according to NIAID's Dr Michael Polis.