In 2007, Merck halted its phase 2b trial of an HIV vaccine after data showed that study subjects given the experimental vaccine had an increased rate of infection. Many researchers at the time blamed the failure of the so-called STEP trial on the viral vector used to deliver the vaccine — a common cold virus called adenovirus-5 (Ad5) — and scientists have been on the look out for alternative vectors ever since.

Just in time for HIV vaccine awareness day, at a meeting this week at the New York Academy of Sciences, Dan Barouch, a virologist at Harvard Medical School in Boston, presented preliminary data from an ongoing phase 1 trial testing a vaccine that uses a related adenovirus-26, Ad26, also a cold virus but one not usually found in humans. He showed that the vaccine was immunogenic at three different doses and only caused mild adverse effects, including fevers, chills and headaches, at the highest dose given.

Based on these findings — the first evidence that an Ad26-based vaccine is safe and effective against HIV in humans — Barouch hopes to test a prime-boost vaccine strategy involving two vectors: Ad35, which has already shown some promise, and Ad26. (Such a combo approach is already undergoing testing against the malaria parasite.)

Notably absent from Barouch’s proposed vaccine was Ad5. Some researchers have speculated that the STEP trial failed because many people already had pre-existing immunity to the virus, but two studies published last year in Nature Medicine refuted that claim. Regardless of the reason, most researchers in the field, including Barouch, are clearly trying to move on.

Image: NIH