It is estimated that 1.8 million people contract HIV every year, and while those rates are lower than when the virus was at its peak in the 1990s, it is still officially the subject of a global pandemic. Developing an effective vaccine has been frustrating scientists for decades due the ability of the virus to rapidly mutate. A team of researchers is now the closest they've ever been to producing a successful vaccine, with an experimental drug moving to a large-scale human trial in southern Africa.

Over the years there have been four experimental vaccines for HIV that have moved to human trials, not including this latest drug. Many other promising compounds have not even made it that far, and of the four that were ultimately tested in humans, just one actually proved efficacious. Called RV144, the vaccine displayed an efficacy rate of just 31 percent in a trial of 15,000 people in Thailand over a decade ago. While somewhat impressive, those rates were not enough to move development of the vaccine forward.

The latest study reports the results of a successful phase 1/2a human trial of a new HIV vaccine across several continents and nearly 400 subjects. The trial used a new approach, testing seven different vaccine formulations in subjects around the world. After giving subjects four doses over one year, the researchers homed in on one particular formulation that seemed to display the greatest anti-HIV immune response. The vaccines were all declared safe for human administration, with only five relatively mild adverse events reported across the entire safety and efficacy trial.

In a related study with rhesus monkeys, the vaccine formulation that was the most successful in humans also displayed great responses in animal experiments. Around two-thirds of the animals displayed complete immunity to SHIV (the simian form of HIV) after six attempts at infection.

The challenge at this stage is to replicate these results in broader human cohorts, and prior work has demonstrated a frustrating inconsistency in moving from primate experiments to human trials. The next stage will be key in revealing whether this is the vaccine breakthrough we've been waiting for, or whether this will be relegated to the growing pile of failed attempts. The upcoming phase 2b trial will take place in southern Africa and encompass 2,600 women designated at-risk for contracting HIV.

Dan Barouch from Harvard Medical School, and one of the researchers leading the prior successful vaccine trials, does urge caution, suggesting that the challenges faced in developing an effective HIV vaccine are unprecedented and excitement should be moderated until this phase 2b trial demonstrates efficacy.

"We eagerly await the results of the phase 2b efficacy trial called HVTN705, or 'Imbokodo', which will determine whether or not this vaccine will protect humans against acquiring HIV," says Barouch.

Other researchers, not involved in this particular study, also urge caution in overemphasizing these early results. George Williams Mbogo, from the Burnet Institute, is optimistic this vaccine could prove useful, even if its results are not 100 percent effective.

"The road to the clinic is still unpredictable since the exact mode of action in humans is still unknown and the 67 percent protection in monkeys might not be replicable in humans," says Mbogo. "The vaccine would go far in shrinking an epidemic expanding by 1.8 million annual new infections, many of which might be resistant to some drugs in clinical use."

The study was published in the journal The Lancet.

Source: The Lancet via SciMex