Scientists said Thursday that a new AIDS vaccine, the first ever declared to protect a significant minority of humans against the disease, would be studied to answer two fundamental questions: why it worked in some people but not in others, and why those infected despite vaccination got no benefit at all.

The vaccine  known as RV 144, a combination of two genetically engineered vaccines, neither of which had worked before in humans  was declared a qualified success after a six-year clinical trial on more than 16,000 volunteers in Thailand. Those who were vaccinated became infected at a rate nearly one-third lower than the others, the sponsors said Thursday morning.

“I don’t want to use a word like ‘breakthrough,’ but I don’t think there’s any doubt that this is a very important result,” said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, which is one of the trial’s backers.

“For more than 20 years now, vaccine trials have essentially been failures,” Dr. Fauci said. “Now it’s like we were groping down an unlit path, and a door has been opened. We can start asking some very important questions.”