For the first time, a vaccine has completely protected monkeys against infection with SIV, a virus related to HIV that infects the animals.

Out of 24 immunised rhesus macaques, 12 had long-term protection, with no signs of SIV a year after they were deliberately infected with the virus.

“In half the monkeys, we saw a dramatic effect on control of the virus,” says Wayne Koff, the scientific director at the International AIDS Vaccine Initiative, which collaborated in the trial. “We see it as a significant advance.”

What made this vaccine different was the use of a live but relatively harmless virus as a host. To make it, Louis Picker at Oregon Health & Science University in Beaverton and colleagues packaged SIV genes into a live virus called a rhesus cytomegalovirus (RhCMV) vector, then injected it under the skin of the monkeys.

They compared its effectiveness with that of conventional vaccines, which deliver SIV genes in harmless adenovirus vectors, similar to those that cause colds.