Seattle ? The first preliminary human testing of a highly anticipated new kind of AIDS vaccine offers tantalizing hints it may ultimately protect against the killer virus.

The study of Merck & Co.’s experimental vaccine is perhaps the most closely watched experiment in all of AIDS research. The approach seems highly effective in monkeys, and many believe it or something similar is the best bet for a shot that could slow the worldwide epidemic, which has already killed 20 million people and infected 40 million more.

Even though the vaccine is only part-way through first-stage safety testing, Merck’s Emilio Emini was asked to update researchers in an address Tuesday at the ninth annual Retrovirus Conference in Seattle.

His bottom line: At this stage, the vaccine appears to trigger the same immune system response in people that it does in newly immunized monkeys, though the volunteers have not been put to the crucial challenge of exposure to HIV.

“We are encouraged,” said Emini, head of Merck’s AIDS vaccine program. “Obviously, the big question is how effective this will be in preventing or mitigating infection. That will have to wait until we get into long-term studies.”

If all goes perfectly, he said, it will be at least five years and probably longer before the vaccine reaches general use.

However, even if the vaccine works, many experts doubt it will stop infection cold the way most vaccines do.

Instead, the hope is to fortify the body’s immune defenses in order to hold the virus in check and prevent disease for many years, perhaps even a lifetime.

The difficulty of developing a vaccine has been one of the biggest frustrations in AIDS research, and experts were enthusiastic Tuesday about the Merck results.

“I would say full speed ahead for this particular research program. These are very encouraging results,” said Dr. David Ho, scientific director of the Aaron Diamond AIDS Research Center in New York City.