An approved vaccine would be about a decade away if the trial and a third study are successful, said officials with the international coalition that is collaborating on the work.
"It is the most promising candidate that we've seen so far," said Sarah B. Alexander, associate director of the coalition, known as the HIV Vaccine Trials Network (search), or HVTN. She cautioned, however, "something better could come along tomorrow."
Volunteers began enrolling last month for the Phase II study, which will eventually give the potential vaccine's three doses to 1,500 people in North and South America, the Caribbean and Australia, the network and Whitehouse Station-based drug maker announced Monday.
The study is using male and female volunteers aged 18 to 45 of diverse racial groups who are at high risk for contracting HIV. Participants will receive counseling about how to reduce their risk of HIV infection, Alexander said.
About a dozen companies and organizations worldwide are attempting to develop an AIDS vaccine. Of the 10 associated with HVTN, the Merck candidate is the first to reach Phase II testing, Alexander said.
Of the two that reached Phase III, the final stage before certification is sought, one was deemed unsuccessful in 2000, and another, led by the U.S. Department of Defense, is enrolling volunteers, she said.
The Merck candidate — the MRKAd5 HIV-1 gag/pol/nef, or trivalent (search), vaccine — is designed to persuade the defenders of each cell, called "killer T cells," to attack HIV when the virus enters the cell. Other vaccines generate an antibody response.
The potential vaccine uses the virus of a common cold, modified so it cannot reproduce or cause people to catch a cold, to transport three synthetically produced HIV genes to the cells.
"We give the body enough of the virus so it can recognize it and create an immune response," but not enough to infect a person, Alexander said.
The Phase I study of the possible vaccine, involving about 1,000 people, "generated strong and durable cellular immune responses against HIV," Merck spokeswoman Janet Skidmore said.
Seattle-based HVTN is funded by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health, an agency of the U.S. Department of Health and Human Services, and comprises more than 25 research institutions worldwide.
Merck shares fell 51 cents, or 1.7 percent, to close at $29.85 in Monday trading on the New York Stock Exchange. The stock, which has fallen sharply since Merck withdrew its Vioxx drug from the market amid safety concerns, has traded in a 52-week range of $25.60 to $49.33.