The 15th International AIDS Conference, dominated by arguments over AIDS drugs and who should pay for them, closed Friday with calls for renewed emphasis on prevention, the oldest weapon in the fight against the pandemic.

"Unless we scale up prevention with the passion and urgency that is being brought to treatment, 'access for all' will remain a dream," Peter Piot, the Belgian physician who heads UNAIDS, said, quoting the slogan of the conference.

A major topic was how to reach the goal of bringing antiretroviral therapy to 3 million AIDS patients in poor countries by the end of 2005. Piot reminded participants that if the rate of new infection isn't slowed, the effort to treat those already infected will be overwhelmed.

"Between today and the deadline for the '3 by 5' initiative, 8 million people will become infected with HIV at the current pace," Piot said. "Without a greatly expanded prevention effort, treatment is simply not sustainable."

Epidemiologists at UNAIDS, which is run jointly by the United Nations and the World Bank, estimate that 37.8 million people worldwide are living with human immunodeficiency virus (HIV), the cause of AIDS, which was responsible for 2.9 million deaths last year.

The delegates at this week's conference, the largest on AIDS to date, heard a distinctly mixed report on progress toward a variety of approaches to prevention, such as vaccines, vaginal microbicides and pills to block HIV infection when a person is exposed to the virus.

Work on vaccines is expanding but so far has led to no useful product. Many experts believe a usable vaccine is at least 10 years off.

Nevertheless, there are now 22 promising vaccines in clinical testing, compared to seven in 2002, when the previous international AIDS conference was held, in Barcelona. Ten of the trials are being held in developing countries.

"The development of an HIV vaccine represents one of the most difficult challenges that modern biomedical science is confronting," Jose Esparza, a virologist with the Bill and Melinda Gates Foundation, told the delegates this week.

Nearly as important -- and perhaps somewhat closer to development -- is a practical microbicide that would protect a woman against infection through sexual intercourse. Researchers are working on numerous compounds that could be applied before sexual contact or used in a cervical collar that would release them for weeks at a time. The key is to give women a protective tool they control, and can use surreptitiously if necessary.

In some parts of southern Africa, 25 percent of women are infected by the time they are 22, said Zeda Rosenberg, who heads the International Partnership for Microbicides, a non-profit group based in Silver Spring. In Kenya and Zambia, adolescents who are married are becoming infected faster than sexually active unmarried teens.

"For women in many parts of the world, being poor, young and married are the most significant risk factors for acquiring HIV infection," Rosenberg said. An effective microbicide, however, would benefit not only them, but, indirectly, their children and male partners.

Also under study is the use of a licensed antiretroviral drug, tenofovir, as an HIV-prevention pill. The National Institutes of Health in Bethesda is paying for an experiment trial among a group of high-risk Cambodian prostitutes.

The next international AIDS conference is scheduled for Toronto in 2006.

Brown reported from Washington.