PHILADELPHIA (AP) — Scientists at Merck & Co.’s vaccine research headquarters in suburban Philadelphia are testing two experimental AIDS vaccines in early human trials.

In previous tests, the vaccines prevented laboratory monkeys from acquiring full-blown AIDS, although they contracted a version of HIV, the virus that causes AIDS.

Experts believe vaccines are the only way to stop the global AIDS epidemic.

“Merck is seen as having one of the broadest programs out there, and is developing a candidate that people look at as one of the lead products,” said Chris Collins, executive director of the New York-based AIDS Vaccine Advocacy Coalition.

Government, academic and industry researchers trying to develop a vaccine over the past two decades have found the goal elusive.

More than a dozen companies are in the hunt with Merck, including Aventis Pasteur, Wyeth, GlaxoSmithKline PLC, Chiron Corp., VaxGen Inc., AlphaVax Inc. and Epimmune Inc.

Merck, which has a 415-acre vaccine research campus near Lansdale, is conducting 10 Phase 1 studies involving 1,300 volunteers around the country.

The company, based in Whitehouse Station, New Jersey, won’t disclose how much it’s spent trying to develop an AIDS vaccine or how many people are working on the project.

Its studies, begun in late 1999, are exploring safety and immune response — and not whether the vaccine prevents people from getting the disease. Results are expected next year.

If the research is promising, studies on the vaccine’s effectiveness in humans could take another three or four years, stretching to 2008 or 2009.

“When could there be an AIDS vaccine? I never answer that question,” said Emilio A. Emini, Merck’s senior vice president of vaccine and biologicals research. “We’ll have to see what the next 10 years bring.”

Merck’s lead vaccine uses a weakened common-cold virus, known as adenovirus, to carry genetic material from the AIDS virus into the body, in the hopes of producing an immune response to the virus.

In studies on monkeys, Merck produced the best immune response when first injecting them with a “naked DNA” inoculation, then boosting it with the adenovirus.

Those monkeys were not free of infection, but the virus remained at low levels and the monkeys did not get AIDS.