One of the HIV vaccines tested in clinical trials may have failed because it spurred not just antibodies active against the virus but gut bacteria as well. Researchers have been studying the results of the HVTN 505 vaccine study, hoping explanations for the trial’s failure may help guide future vaccine development.

The study participants were given an initial “prime” vaccine followed by a booster shot.

The investigators found that the vaccine gave rise to antibodies that recognized a protein on the surface of HIV called gp41. Nevertheless, the antibodies were not able to neutralize the virus. This may be because these antibodies also recognized gut bacteria, and such multifaceted reactivity ultimately decreased the vaccine’s effectiveness against HIV.

To read a press release on the research, click here.

To read the study abstract, click here.