Merck halted its Phase IIb clinical trials of an HIV vaccine last week, after data showed a lack of efficacy. According to a news release from the National Institute of Allergy and Infectious Disease (NIAID), "the vaccine cannot be shown in this trial to prevent HIV infection or affect the course of the disease in those who become infected with HIV."

This is the same vaccine The Scientist highlighted this month in its feature on promising lines in HIV research. Merck's vaccine, based on an adenovirus vector, was the furthest along in clinical trials. During my reporting of the article, which went to press mid-August, some expressed concerns over the potential pitfalls of pre-existing immunity to adenovirus that might compromise its efficacy.

Whether this is the reason the vaccine failed is unknown. It's also unclear what will happen with NIAID's upcoming clinical trials on a different adenovirus vector vaccine against HIV. According to a press statement from the International AIDS Vaccine Initiative (IAVI), which is participating in these upcoming trials, "the organization has already heard from NIAID that it is reviewing the data from the Merck trial to ensure that the design of its upcoming PAVE trial remains appropriate." IAVI has postponed a supporting trial to the PAVE study until more information on the Merck trials becomes available.