Just eight months after a free clinic opened in Karonga Town in June 2005, the death rate in a rural area 80 km (50 miles) away had fallen enough to be detected at the general population level, they wrote in the Lancet medical journal.

"I think people didn't expect to see an effect that quickly," investigator Judith Glynn of the London School of Hygiene and Tropical Medicine told Reuters.

"Previous clinical studies have shown improved survival but we have now shown this translates into a clearly measurable effect, even in a rural area, and surprisingly early."

Malawi, with a population of 13 million, is one of the countries hardest hit by HIV/AIDS. The pandemic accounts for 59 percent of deaths among those aged between 15 to 59 years.

Since 2004, however, Malawi has been able to start offering free antiretroviral therapy to some patients, with support from the Global Fund to Fight AIDS, Tuberculosis and Malaria.

Glynn and colleagues found overall mortality in their study population of 32,000 fell by 10 percent in the eight months after the free clinic opened and plunged 35 percent among adults living near the area's only tarmac road.

Commenting on the results, Mattias Egger and Andrew Boulle of the University of Cape Town said the rapid impact reflected the fact that the sickest people -- those most likely to die -- were the first to receive drugs.

As treatment continued, however, new patients would be less ill and the benefits might not be seen so quickly, they added.

The marked variation in reductions in mortality rates between those near and far away from the main road also highlights potential inequalities in getting treatment to all those who need it.

Malawi has about 800,000 deaths from AIDS every year but the country has been one of the more successful at rolling out free treatment to patients.

Worldwide, some 3 million people in poor countries now receive antiretroviral therapy, thanks to funding from the Global Fund and the U.S. government, helped by falling prices of both branded and generic medicines.