And scientists are completely baffled:

A new AIDS vaccine tested on more than 16,000 volunteers in Thailand has protected a significant minority against infection, the first time any vaccine against the disease has even partly succeeded in a clinical trial.

Scientists said they were delighted but puzzled by the result. 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.

"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, 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," he went on. "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."

[snip]

Col. Jerome H. Kim, a physician who is manager of the army's H.I.V. vaccine program, said half the 16,402 volunteers were given six doses of two vaccines in 2006 and half were given placebos. They then got regular tests for the AIDS virus for three years. Of those who got placebos, 74 became infected, while only 51 of those who got the vaccines did.

Although the difference was small, Dr. Kim said it was statistically significant and meant the vaccine was 31.2 percent effective.

So what we're looking at here is a 30% effectiveness rate. How bizarre. Adding confusion is the fact that those who did become infected with the vaccine did not have lower viral loads than those who became infected with the placebo, something that is generally expected with vaccine trials:

The most confusing aspect of the trial, Dr. Kim said, was that everyone who did become infected developed roughly the same amount of virus in their blood whether they got the vaccine or a placebo.

Normally, any vaccine that gives only partial protection -- a mismatched flu shot, for example -- at least lowers the viral load.

That suggests that RV 144 does not produce neutralizing antibodies, as most vaccines do, Dr. Fauci said. Antibodies are long Y-shaped proteins formed by the body that clump onto invading viruses, blocking the surface spikes with which they attach to cells and flagging them for destruction.

Instead, he theorized, it might produce "binding antibodies," which latch onto and empower effector cells, a type of white blood cell attacking the virus.

Obviously, this trial is not the Holy Grail. But it is indeed interesting and compelling new data that will have an obvious effect on future trials. Combining two failed vaccine candidates was a HIGHLY controversial idea, but it appears to have paid off -- at least in some small fashion.