The ultimate weapon against AIDS would be a vaccine to prevent infection with the HIV virus. Until now, though, every test of experimental vaccines has failed.

But results from the largest-ever vaccine trial gave new hope to scientists that, in principle, a vaccine could work someday.

Researchers said a vaccine tested in more than 16,000 people in Thailand protected a significant number of them against the wily virus, which destroys the immune system. The risk of HIV infection among the vaccinated people was cut by 31 percent.

Read more: NPR