HIV vacc failure may be linked to Ad5 vector

USA-based drug major Merck & Co says that post hoc analysis of data from trial of its discontinued HIV vaccine candidate, V520, indicates that vaccinated participants with pre-existing immunity to the adenovirus 5 vector used in the product were more likely to become infected with HIV than those "vaccinated" with placebo. The firm reiterated that V520 alone could not cause HIV infection, because it contains genes for viral antigens designed to elicit an immune response rather than the complete HIV genome required for replication.

Keith Gottesdiener, vice president of vaccine and infectious disease clinical research at Merck, suggested that earlier exposure to Ad5 may have led patients subsequently given the vaccine to mount an elevated immune response, and thereby increase the population of circulating immune cells that are susceptible to HIV.

This idea was leant further credence by Bruce Walker, a researcher at the Harvard Medical School, who said that the repeated immunizations used to deliver the vaccine could have resulted in transient periods in which patients were more at risk of infection.