The U.S. National Institutes of Health (NIH) has announced that a clinical trial of an experimental HIV vaccine has been halted, following a report from a safety monitoring board. The independent data and safety monitoring board found during an interim review that the vaccine does not prevent HIV and does not reduce viral loads in people who have HIV.

According to the NIH the study, called HVTN 505, has been running since 2009. 2,504 volunteers in 19 U.S. cities were chosen to undergo a series of immunizations that researchers hoped would prevent HIV infection and reduce viral loads in those infected (or receive a placebo). The volunteers were all either men who have sex with men or transgender people who have sex with men.

After reviewing data from the study this week, the monitoring board found that study participants who received the vaccine were just as likely to contract HIV as those who had not. In fact, more study participants who received the vaccine became infected with HIV than those who received a placebo, though the difference was not statistically significant. The vaccine also failed to reduce viral load among study participants who contracted HIV.

Due to the findings, the National Institute of Allergy and Infectious Diseases (NIAID) has stopped administering the vaccinations. Researchers will continue to follow current study participants, who will be contacted and informed of whether they received the vaccine or the placebo.