A booster shot of an experimental HIV vaccine given four years after the first round of three shots prompted a significant immune response in HIV-negative study participants, Reuters Health reports.

Publishing their findings in PLOS ONE, Spanish researchers from the RISVAC02boost study examined the effects of the booster shot.

In a previous study, 24 HIV-negative individuals received three shots of a modified virus that expressed specific HIV antigens, which are viral proteins that prompt an immune response. This study found that the vaccine was safe, well-tolerated and prompted a moderate immune response that was sustainable over time—specifically an immune-cell­­­-based response in 75 percent of the participants and an antibody-based response in 95 percent of them.

Four years later, 13 of the original participants received a booster antibody shot. Before receiving the shot, 12.5 percent of them had detectable CD4 and CD8 cells that were specific to HIV, compared with 45 percent of them two and four weeks after the booster.

The HIV antibody response peaked dramatically two weeks after the booster and declined rapidly through week 12.

Before receiving the booster shot, none of the study participants’ serum samples could neutralize HIV, compared with 77 percent two weeks after the booster. Such neutralizing ability declined through 12 weeks after the booster.

All the participants experienced at least one adverse health event after the booster shot. Ninety-six percent of the health events were grade 1 (mild) and 4 percent were grade 2 (moderate).

The researchers concluded that the experimental vaccine may one day serve as a component of an effective vaccine regimen.

To read the Reuters Health report (free registration with Medscape is required), click here.

To read the study, click here.