http://www.nytimes.com/2008/08/04/health/04aids.html

Can a pill a day help prevent infection from H.I.V., the virus that causes AIDS?

No one knows. But researchers in a number of countries are conducting trials and planning others to test the unproven strategy that a daily pill, or a combination of drugs, can prevent H.I.V.

By mid-2009, more people will be enrolled in such trials than in all of those for H.I.V. vaccines and microbicides, the AIDS Vaccine Advocacy Coalition said in a report issued here on Sunday at the start of the 17th International AIDS Conference.

Initial findings of the safety and effectiveness might come early next year, although researchers do not know how they will compare to the disappointing results of recent tests of H.I.V. vaccines and microbicides, chemicals that women can put in their vagina to prevent H.I.V. infection.

In the face of those bleak findings, some AIDS experts say testing the prophylactic use of antiretroviral drugs, called PrEP, for pre-exposure prophylaxis, is now the most promising research in H.I.V. prevention efforts as scientific investigation of vaccines and microbicides continues.

The Centers for Disease Control and Prevention, which released a report on Saturday showing that the number of people newly infected with H.I.V. in the United States in recent years was 40 percent higher than has long been reported, said that PrEP was among the strategies that needed to be developed to substantially reduce the incidence of H.I.V. An estimated 2.7 million people become infected each year worldwide.

"We cannot wait for the study results to begin to prepare for the optimal use and delivery of PrEP," said Pedro Goicochea, an investigator in a PrEP study under way in Peru and Ecuador. "Instead, we should look ahead to consider all of the possible outcomes of these trials and make real plans for making PrEP available to those who can benefit from it, as quickly and safely as possible if it is proven effective."

The United States Agency for International Development, the C.D.C. and the National Institutes of Health are paying in part for all of the trials. The Bill & Melinda Gates Foundation is paying for part of two. The organizations have undertaken the trials because of the success in giving antiretroviral drugs to pregnant women to prevent H.I.V. infection in their infants and drugs to prevent malaria.

In 2007, Family Health International completed a similar study of the antiretroviral drug tenofovir for H.I.V. prevention among young women in Ghana, providing the first data showing such use was both safe and acceptable among uninfected users. But the study did not indicate if PrEP was effective in preventing new infections.

Studies in a small number of nonhuman primates hint that PrEP can reduce the risk of infection of a simian form of H.I.V. in nonhuman primates.

Initial PrEP studies are testing tenofovir, either alone or combined with another drug, emtricitabine.

Infected people taking these licensed drugs have shown limited side effects like nausea, vomiting, diarrhea and intestinal gas. But the safety of the drugs must be established among noninfected people as well as among participants who become infected in the study.

Trial participants include gay and bisexual men, heterosexual men and women, couples in which one partner is H.I.V. infected, and sex workers.

Up to 15,000 people are expected to be participating in trials by mid-2009. Sites of the trials include Botswana, Brazil, Ecuador, Kenya, Malawi, Peru, South Africa, Tanzania, Thailand, Uganda and the United States.

Male circumcision has proved effective in a significant percentage of H.I.V.-negative people in trials in Africa.

Because PrEP might also work in a significant percentage of uninfected users, the advocacy coalition and other researchers say now is the time for governments, health officials, donors, researchers and advocates for AIDS research to prepare for the maximum public health benefit if the pill-a-day strategy works.

One reason for the immediacy, they say, is that even if a PrEP strategy proved successful, it would not be a silver bullet. PrEP would need to be combined with standard prevention measures like safer sex practices, use of condoms and clean needles, and counseling.

Governments would need to prepare for global procurement of PrEP, arrange adequate staffing to deliver the drugs and provide counseling and determine who will pay for the costs, said the coalition, which describes itself as an international nonprofit, community- and consumer-based organization.

Provisions would also need to be made for participants who do become infected, and findings from a number of trials would be needed before PrEP could be widely recommended.

Results may be hard to interpret because the tests are being conducted in different ways and so may produce varying findings, the advocacy coalition's report said. Statisticians could face challenges in pooling the data from the trials.

The report also said that the initial findings were likely to raise questions needing further study, like whether taking pills intermittently — like just before a sexual act — can be effective. The report is available online at www.avac.org/prep08.pdf.

Researchers conducting the studies said they were taking longer than initially expected. Among the problems are recruiting volunteers and addressing the real or perceived stigmatization of those who participate.