The only clinical trial in the world investigating whether an HIV vaccine works in people at high risk for AIDS - including 40 in Rochester - has been halted early.

The Merck vaccine didn't prevent HIV infection or slow the disease in those who became infected, according to preliminary results of this latest international study.

This setback in the fight to prevent AIDS may have wider implications. Most other HIV vaccine studies in the pipeline are taking a similar approach, which is to prompt the body to produce more of a crucial immune cell called killer T cells.

"It clearly is a disappointment," Dr. Michael Keefer, director of University of Rochester Medical Center's HIV vaccine trials unit, said Wednesday. "Frankly, the Merck vaccine looked as good as anything we had in the test tube."

The STEP study, also known as the HVTN 502 study, began in December 2004 and included 3,000 adults who were HIV-negative when enrolled but who were considered at high risk for the infection that causes AIDS. Participants included both men and women. The UR researchers recruited gay men. The study took 18 months to be fully enrolled, so it wasn't scheduled to end until mid-2009.

The study was stopped in late September after the first evaluation of interim results found that about 3 percent of study participants had developed HIV, both among those who received the vaccine and among those who received a placebo, or fake vaccine. Neither the volunteers nor the researchers knew who received which until the end of the study. Keefer said he wasn't sure whether any Rochester participants developed HIV.

The early global results found that of 741 volunteers who received the vaccine, 24 developed HIV. Among 762 volunteers who received a placebo, 21 developed HIV.

HIV spreads through unprotected sex and through shared needles among injected-drug users. No investigational vaccines can cause HIV infection because no living or killed virus, or pieces of virus, are used.

All study participants were counseled on how to reduce their risk of getting HIV and given various supplies, such as condoms. Those who became HIV-infected during the study were referred for evaluation, medical care and treatment and offered continued immunological and virological follow-up for an extended period of time.

At this time, there is no proven cure or vaccine for HIV. There are highly effective treatments to keep people with HIV healthy longer and to delay the onset of AIDS, which stands for acquired immune deficiency syndrome.

University of Rochester Medical Center was among 27 STEP study sites in North America, South America, the Caribbean and Australia. The HVTN 503 or Phambili study, a trial of the same investigational vaccine among heterosexual adults, started in January in South Africa and was expected to be that country's largest with 3,000 participants. It also has been halted.

Earlier this decade, in the first wave of HIV vaccine attempts, researchers tried to develop an injection that would stimulate antibodies against the virus. That approach hasn't worked so far.

Keefer said other investigational vaccines show promise. URMC and other research centers around the world expect to start a new clinical trial in 2008 called PAVE 100 that would involve 9,000 participants at high risk for HIV.

URMC also is running about a dozen studies of potential HIV vaccines that showed promise in studies of monkeys. The so-called Phase I studies are small and test the safety of the vaccines in people. . Vaccines that pass that test can eventually move to Phase IIB - like the STEP test - to examine whether the vaccine prevents HIV in high-risk individuals. Positive results would lead to Phase III trials and, potentially, a licensed vaccine.

Steven Price, director of prevention services at AIDS Rochester, said Wednesday, "We're disappointed that the vaccine was not working, but are encouraged by the tenacity of those doing the difficult work of finding a vaccine to combat HIV/AIDS. AIDS Rochester will continue to support vaccine research both locally and globally."