Two international HIV vaccine trials have been discontinued today due after an interim analysis this week indicated that the vaccine candidate was not effective in preventing transmission of the virus or in lowering the viral load of those infected.

The “STEP” study was designed by Merck and the HIV Vaccine Trials Network (HVTN) to test the efficacy of the MRK-Ad5 vaccine in Australia, Brazil, Canada, the Dominican Republic, Haiti, Jamaica, Peru, Puerto Rico and the United States.

Its companion, the “Phambili” study, had enrolled 799 individuals in South Africa to test the effectiveness of the Merck vaccine, and was funded by the HVTN and the South African AIDS Vaccine Initiative. While the STEP study primarily enrolled men who had sex with men, the Phambili participants were mostly heterosexuals at high risk for HIV infection.

Enrollment and immunizations for both studies have been ceased. Mitchell Warren, executive director of the AIDS Vaccine Coalition, says that the discontinuation of these studies is an unfortunate yet necessary step in creating a vaccine for HIV/AIDS.

“It must be seen for what it is: the failure of a product to show efficacy in a specific trial,” he says. “Clinical testing of AIDS vaccines is a scientific process, and, while this is a disappointment, it is in no way the end of the search for an AIDS vaccine.”