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The road to a vaccine to protect against HIV and AIDS has run into a lot of dead ends, but a new study in monkeys suggests a new path may have been found.  Researchers say two new experimental vaccines partially protected monkeys from an HIV like infection, reducing the likelihood of contracting the disease by 80% to 83%, compared to the placebo.

Both studies, published online Wednesday in the journal Nature, tested severalSimian Immunodeficiency Virus (SIV) vaccine regimens on 40 rhesus monkeys each.  Researchers found the vaccines provided some protection to monkeys that were exposed to an extremely virulent, hard to neutralize strain of SIV. Not only did the vaccines reduce the chance of infection, but for monkeys that became infected, it substantially reduced the amount of virus in their blood.

“This type of protection in terms of blocking acquisition of this tough virus challenge has not been seen before by any vaccine in this animal model,” said Dr. Dan Barouch, associate professor of medicine at Harvard Medical School and lead study author.  “So I think there is reason to be optimistic; in fact, I think we are more optimistic than ever before about the possibility of an HIV vaccine for humans.”

Barouch says SIV is very similiar to HIV and will give researchers insight and clues into how to design an HIV vaccine for humans. “Clinical trials have to be done and there’s no guarantee, but the SIV model in rhesus monkeys is the best preclinical test of vaccine strategies before embarking on HIV vaccine clinical trials in humans.”

The dose of the virus used in the animals studies was 100 times more infectious than what is seen in human exposure, according to Barouch.  Once vaccinated, the monkeys received multiple exposures to SIV over a certain period of time.  Eventually most of them did become infected, but the results are still promising, Barouch tells CNN.  “The data show these vaccine candidates achieved better protection in these preclinical models than some previously tested candidates and so therefore we are optimistic in moving the novel lead HIV candidates into clinical trials.”

The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, conducts and supports HIV vaccine studies around the world. They helped fund the research.

“What makes this paper so remarkable is that it represents the first time we’ve identified the likely correlates for both protection from HIV acquisition and control of viral replication,” said Carl Dieffenbach, director of the NIAID Division of AIDS.  “This will have important implications for the next round of HIV vaccine clinical trials.”

Researchers now plan to test a human-adapted version of one of the vaccine combinations in healthy adults here in the U.S. and internationally. Barouch says manufacturing has already begun.  “It is not possible to predict how this vaccine will perform in humans, but this study advances our understanding of AIDS pathogenesis

and immunity,” he says.