Researchers felt boosted by the results of an AIDS vaccine clinical trial released on Thursday, which showed for the first time that a small but significant portion of volunteers were protected against infection.

Results of the AIDS vaccine trial, which involved more than 16,000 volunteers in Thailand, left scientists “delighted but puzzled,” according to The New York Times.

“The vaccine -- a combination of two genetically engineered vaccines, neither of which had worked before in humans -- protected too few people to be declared an unqualified success. And the researchers do not know why it worked,” reportedthe Times.

The vaccine, known as RV 144, proved 31.2% effective in the clinical trial.

The trial’s backers include the National Institute of Allergy and Infectious Diseases, the United States Army, the Thai Ministry of Public Health, and the patent-holders for the two parts of the vaccine, Sanofi-Pasteur and Global Solutions for Infectious Diseases.