Scientists have developed a new vaccine that partially protects monkeys from an infection much similar to HIV, a breakthrough which they say could be a new weapon in the battle against AIDS.

Researchers at the University of Utah School of Medicine found that rhesus monkeys which received the new vaccine were 80 to 83 per cent less likely to get infected with the Simian immunodeficiency virus (SIV), a virus which is closely related to HIV, when exposed to it.

The experimental vaccine was engineered to trigger the monkeys’ immune systems to fight the virus. Researchers often study the effects of an SIV vaccine in monkeys as a way to understand how to develop an HIV vaccine in humans.

This is an important step toward an HIV vaccine, said Dr. Adam Spivak, an HIV researcher at the university.

“This study demonstrates that the immune system can be prepared to respond to, and partially control, viral infection that mimics HIV-1 transmission in humans,” Spivak was quoted as saying by LiveScience. HIV-1 is the type of the virus that causes most infections in people.

At the start of the research, published in the journal Nature, 40 monkeys were injected with either the experimental vaccine or the placebo vaccine, and then given a booster shot at six months.

The monkeys were then exposed to SIV six times through intra-rectal administration of the virus six months later.

After the virus was given the first time, about 75 per cent of monkeys in the placebo group became infected, whereas only 12 to 25 per cent of those given the vaccine developed infections. After the third exposure, about half the animals that received the experimental vaccine were infected and by the sixth time, almost all the animals were infected.

Still, those were on the experimental vaccine had a lower amount of virus in their blood than those given the placebo, the researchers found.

“This shows that the vaccine provided some resistance to primary infection,” Spivak said.

The results also showed that one vaccine ingredient — a protein called Env, which is involved with binding the virus with a cell — was key in helping to block the virus from entering the monkey’s cells, the researchers said.

Although efforts were made in past studies, including the 2009 Thailand study that found one vaccine reduced the risk of contracting HIV by nearly a third, other recent trials of vaccines have not been as successful.

“There have been a number of human trials using a similar vaccine that have not worked, or there wasn’t enough evidence to show if the vaccine primed the immune system appropriately,” Spivak said.

But the new findings will provide a template for vaccine design and for measuring its effectiveness for future studies, he said.

“Researchers have shown that a vaccine can create an immune response. That, in itself, is exciting,” he added.