Changes in sexual behaviour have slowed the spread of HIV in Zimbabwe, researchers say.

The prevalence of HIV fell from 23 per cent of the population in 1998 to 20.5 per cent in 2003, according to a study published in Friday's issue of the journal Science.

"Although we can't say for certain, fear of HIV and AIDS may have influenced this change in behaviour, with Zimbabwe's well educated population, good communications, and health service infrastructure, all combining to create this effect," said Simon Gregson of Imperial College London, who led the research.

More than 25 million Africans, including this girl, already have the virus that causes AIDS.

Limiting partners, delaying sexual activity and using condoms are all possible reasons for the decline, he said.

Gregson's team interviewed 9,454 people in eastern Zimbabwe for the study, which offers the first evidence of a decline in HIV prevalence in southern Africa.

Among men surveyed aged 17 to 54, HIV prevalence fell from 19.5 per cent to 18.2. It declined from 25.9 per cent to 22.3 per cent in women between the ages of 15 and 44.

There was no evidence men increased their use of condoms with regular partners but "for men, consistent condom use in casual partnerships lessened the risk of HIV infection," the researchers wrote.

They note the reports need to be interpreted cautiously, because migration, deaths among people infected with HIV and underreporting of high-risk sexual behaviour could also explain the decline.

Delaying sex and condom use in Zimbabwe may mirror behavioural changes in Uganda, where there has also been a long-term decline in HIV prevalence, the study's authors said.

"There is also growing evidence of declining prevalence in Kenya, Burkina Faso, Cambodia, and Haiti," Richard Hayes and Helen Weiss of the London School of Tropical Medicine and Hygiene said in an accompanying commentary.

"However, the overall picture in sub-Saharan Africa is one in which most countries now have 'stable' epidemics, where AIDS-related mortality is matched by the rate of new infections, and in some southern African countries, the prevalence of HIV continues to increase."

Another five million people were infected in 2005, bringing the total number living with HIV/AIDS to about 40.3 million worldwide, according to UN AIDS. Sub-Saharan Africa remains the most affected area in the world.