A change in the sexual behavior of young people in Zimbabwe probably caused a sharp decline in HIV virus infections in the south African country, according to a study published in the United States.

A reduction in casual sex among experienced men and women, and the delay of others' first sexual experiences, drove a steep fall in the rate of infections of HIV, the virus which causes AIDS, said the study.

An international team of researchers from Britain, Zimbabwe and South Africa which surveyed the behavior and infection rates of people in eastern Zimbabwe between 1998 and 2003 observed a 49 percent reduction in HIV infections among women aged 15 to 24.

Among men between 17 and 29 years of age, the rate dropped 23 percent.

"A key reason for this decline appears to be the reduction in the number of casual sexual relationships, although there was also a delay in the onset of sexual activity and increases in condom use prior to the time of the study may also have contributed," said researcher Geoffrey Garnett of Imperial College London.

The study, to be published in the February 3 issue of Science magazine, notes the difficulties of linking perceived declines in HIV infection with behavior due to the virus's long average incubation period. Behavioral changes can long precede the detection of an infection decline, it noted.

However, it postulated that a significant fall in infection rates in Uganda in the 1990s came after sexual behavior changes.

"Our data suggest that the changes in behavior occurring in Zimbabwe are similar to those underpinning the long-term decline in HIV prevalence in Uganda, i.e., a delay in age at first sex and a reduction in casual sex, but that consistent condom use with casual partners has also contributed," the report said.

The study is important especially for sub-Saharan Africa, where AIDS rates are the highest in the world. In Zimbabwe in 2003, an estimated 1.8 million of the country's total population of 12 million were infected with HIV.

A separate Science study also released Thursday noted that the entire sub-Saharan Africa region has more than 25 million HIV/AIDS sufferers, and that almost five million new infections took place in 2005.

"Give this grim scenario, the significant decline in HIV prevalence in Zimbabwe ... provides clear evidence of a reduction in HIV prevalence associated with behavior change in this region of Africa," said authors Richard Hayes and Helen Weiss of the London School of Hygiene and Tropical Medicine.

They said that Zimbabwe could be added to a short list of countries with substantial drops in AIDS infection, the others being Uganda and Thailand.

They also noted evidence of declines in Kenya, Burkina Faso, Cambodia and Haiti.

In the Zimbabwe study, researchers from Imperial College London and Zimbabwe's Biomedical Research and Training Institute studied 9,454 people recruited from two household censuses, the first conducted between 1998 and 2000, and the second between 2001 and 2003.

They found that overall HIV infections fell from 23 percent to 20.5 percent during the five-year period. In men aged 17 to 54, it declined from 19.5 percent to 18.2 percent, while in women aged 15 to 44, it declined from 25.9 percent to 22.3 percent.

But the sharpest declines were in younger people, especially those with basic schooling.

Dr Simon Gregson, from Imperial College London, who led the research, said: "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."

New research shows much smaller doses of vaccines than previously thought may be needed to combat pandemics of avian flu, tuberculosis and SARS, Canadian scientists told AFP.

The research by a team at the University of British Columbian in Vancouver could mean more shots will be available.

The team led by Wilfred Jefferies found standard doses could be reduced 100-fold and still be safe by adding an immune system booster commonly found in the human body.

The study showed that when the immune system molecule was added at low doses to vaccines and injected into mice exposed to rabies, measles and smallpox, an immune response kicked in to protect them from the infections.

The so-called TAP molecules deliver peptides inside cells from the body's immune system or vaccines. If the transporter is switched off, as in cancerous cells, disease fighters cannot be moved where they need to go within the cell to do their job.

The results of the seven-year study, co-authored by Timothy Vitalis, were published in a recent issue of the American journal Public Library of Science Pathogens based in San Francisco.

In the article, the scientists said: "This new paradigm is shown to be applicable to many viruses, including poxviruses, and could significantly advance the creation of new vaccines and improve those that already exist."

This could prove especially important if the H5N1 avian flu, expected by many to be the next worldwide pandemic, begins to pass between humans.

Bird flu has killed some 85 people since emerging in Asia in 2003. Experts have said it could kill millions if it mutates into a form easily passed between humans.

It could take up to six months to develop a vaccine should human-to-human transmission occur. With smaller dose sizes, a hundred times more vaccine shots could be made within the same period of time.

"We're hoping it will be incorporated into a number of different vaccines," Jefferies told AFP. "It's powerful because we think it can make general vaccines much better."

Smaller doses will also make some vaccines, including that for smallpox and anthrax, less toxic and therefore better tolerated by those with weaker immune systems, Jefferies said.

HIV patients, in particular, could benefit, as well as up to an estimated 20 per cent of the population who react adversely to vaccines.

"Some pathogens don't currently have effective vaccines," said Vitalis. "I hope the technology can be used to solve some of the needs in controlling infectious diseases."

The research cost some 1.5 million Canadian dollars and was primarily funded by the Canadian Institutes of Health Research.

Jefferies first imagined the possibility of this application for vaccines after noticing that TAP is turned off in cancer cells, meaning that the body's natural immune defense does not recognize that it needs to attack these cells.

He found that when TAP was added to the cancerous cells the immune system suddenly "switched on" and fought back.

"TAP is a key molecule in the creation of immune responses," said Jefferies. "This molecule fought viruses when it was found in normal amounts in the cell. I thought, 'If a little bit is good, is a lot (of TAP) even better?"

Researchers said they are not sure why the TAP molecules are turned off in cancer cells, but hope further study may eventually lead to a new cancer treatment too. However, that is still years away.

Jefferies said the team is currently trying the method out with prototype vaccines that could lead to clinical trials in the next two years, depending on funding.

There has been considerable international interest in the many uses for low dosage vaccines, he said, with "lots of e-mails coming from scientists, companies and housewives."