The search for an effective vaccine to protect against HIV has hit yet another roadblock. A major clinical trial on what was once deemed a potential blockbuster strategy has been halted by federal researchers, after it was determined that the approach didn't prevent infection among study participants.

The trial, which kicked off in 2009 and included around 2,500 participants, set out to evaluate a vaccine called HVTN-505. The vaccine relied on a weakened version of a common cold virus, called Ad5, which operated as a vehicle to carry key HIV genes into the body and prime the immune system to defend against the illness. It's an approach that's been tried unsuccessfully before, namely in a 2007 trial, called STEP, that also ended early.

"The major message is they were not protected"

In fact, both trials appear to share yet another troubling similarity: participants who received the vaccine appeared slightly more likely to contract HIV than those who received placebo. In this latest trial, 41 participants receiving the vaccine became infected with HIV, compared to 30 in the placebo group. The difference isn't statistically significant, but study leaders said they can't rule out the possibility that this vaccine actually increased an individual's risk of HIV. "It's clear evidence the vaccine didn't work and may in some fashion put them at greater risk — we don't know that," study leader Scott Hammer, M.D., told NPR. "The major message is they were not protected."

Though study participants won't receive further vaccines, researchers will continue to analyze data collected from the study and plan to track participants for the next five years. They're hoping to glean more insight into where the approach went wrong, and determine whether any tweaks might make it viable for future study.

Despite this latest failure, other research teams continue to work on novel new approaches to an HIV vaccine. One novel project, described earlier this month in the journal Nature, would trigger the production of unique antibodies — known as broadly neutralizing antibodies — that are capable of combating the HIV virus even as it evolves inside the body.