People in Zimbabwe, a country hard hit by AIDS, have been able to slow the spread of the virus by changing their sexual behavior, including limiting partners, delaying sex and using condoms, researchers said on Thursday.

They found the spread of HIV dropped the most among women aged 15 to 24, with a 49 percent drop in prevalence. There was a 23 percent drop among men aged 17 to 29.

Behavior is almost certainly a major factor, the researchers reported 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 behavior, with Zimbabwe’s well-educated population, good communications, and health service infrastructure all combining to create this effect,” said Dr. Simon Gregson of Imperial College London, who led the research.

Their study of 9,454 people, some of whom were first interviewed in 1998, showed the overall prevalence of the human immunodeficiency virus fell to 20.5 percent by 2003 from 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 Geoffrey Garnett of Imperial College, who also worked on the study.

AIDS remains a global pandemic, with more than 40 million infected with the incurable and fatal virus. It killed more than 3 million people in 2005 and infected 5 million new patients, according to UN data.

Africa is by far the worst-hit region, with most cases spread sexually. The UN AIDS agency UNAIDS has noted a decline in three sub-Saharan African countries: Kenya, Uganda and Zimbabwe.

UNAIDS credits changes in sexual behavior and notes that condom use exceeds 80 percent in Zimbabwe.

“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,” Gregson’s team wrote.

Peter Ghys and colleagues at the United Nations Joint Program on HIV/AIDS in Geneva, and colleagues said almost 30 million new HIV infections worldwide could be prevented in the next 10 years with stronger prevention programs in low- and middle-income countries.

For each $3,900 spent to prevent one new infection, about $4,700 in treatment and care costs could be avoided, they wrote in a second study published in Science.

“Our analyses suggest that both national governments and donor countries would be well advised to ensure that prevention programs are scaled-up as soon as possible,” they wrote.