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American scientists gathering for the 17th International AIDS Conference say vaccines remain an important area of study in attacking the disease, despite recent setbacks, and they are also excited by other potential preventive steps such as a daily dose of antiviral drugs.

The past two years have featured frustrations on several fronts in the battle to contain HIV, the virus that causes AIDS, with researchers coming up empty on tests of vaccines, microbicide gels, diaphragms and a herpes treatment. This has spurred them to refocus on some basic questions: What makes an effective immune defense against AIDS, and how can scientists create neutralizing antibodies that can block HIV from infecting people?

AIDS, which exploded in 1981 as a sexually transmitted and blood-borne disease, has killed an estimated 25 million people, and 33 million world-wide are currently infected with HIV.

Treatment programs now reach three million people, prolonging lives, lowering virus levels in blood and reducing transmission. But despite efforts to block the epidemic by promoting abstinence, monogamy, condom use and circumcision, there are still 2.7 million new HIV infections a year -- about 7,400 a day.

In interviews ahead of the conference, which starts in Mexico City on Sunday, researchers and donors said fresh ideas to reverse this trend abound. Among the approaches drawing the most interest are the antibody work of Scripps Institute's Dennis Burton; investigations by Harvard University's Bruce Walker into "elite controllers" whose bodies naturally keep HIV in check; and studies by Harvard researcher Dan Barouch of "designer" adenoviruses that could power a vaccine without the problems that doomed Merck & Co.'s experimental vaccine last fall.

"Anybody who sees this as the end of vaccine trials doesn't understand what's going on," said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, a unit of the National Institutes of Health in Bethesda, Md.

In Seattle, the Bill and Melinda Gates Foundation recently boosted its already formidable investment in HIV prevention to a total of $1.49 billion. The war chest is divided between research on new prevention tools -- vaccines, microbicide gels, pills -- and expanding use of existing ones, such as condoms.

Near-term excitement is focused on testing whether a daily dose of AIDS antiviral drugs can prevent HIV infection. Known as "pre-exposure prophylaxis," or PREP, the concept is being tested in about 20,000 volunteers around the world.

"If you ask me what will come first," Mr. Gates said in an interview, "something like PREP has a good chance of becoming available before we have a 100%-efficacious vaccine. The challenges are a little less daunting. If we have that tool, it could have a very big impact."

Mr. Gates recently increased to $93 million his foundation's investment in PREP trials.

Robert S. Grant of the Gladstone Institute and the University of California, San Francisco is using funds from the Gates foundation and the NIH to test a daily pill of Truvada, a drug cocktail combining Viread and Emtriva by Gilead Sciences in high-risk volunteers in Brazil, Peru, Ecuador and the U.S. Results are scheduled for 2010, but could come earlier.

"We have never seen in HIV an idea tested in such a large diversity of populations," Dr. Grant said in an interview from Rio de Janeiro.

Zeda Rosenberg, chief executive of the Washington nonprofit, International Partnership for Microbicides, says new-generation products include a vaginal gel based on Gilead's tenofovir, a vaginal ring containing dapirivine from Johnson & Johnson's Tibotec unit, and a microbicide using Pfizer Inc. AIDS drug maraviroc.

Despite widespread disillusion, Ms. Rosenberg said the success of a microbicide is simply "an engineering problem, getting the right drug to the right place at the right time."