Why did the Merck vaccine not work?
A detailed examination of the reasons the Merck ad5 vaccine failed to work in the STEP trial awaits investigations into the exact anti-HIV immune responses seen in trial subjects, and especially into whether there was a ‘mismatch’ between these immune responses and the viruses actually generated in people. However, this mismatch – caused by HIV’s furious rate of replication and genetic diversity – seems to be the most likely explanation. Otto Yang of the University of California, Los Angeles explained why he thought CD8/CTL responses appeared insufficient to contain HIV infection in a presentation to the HVTN Conference in November 2007 (Yang 2007). It was unquestionable that CTLs could contain HIV in vitro and in some people in vivo, he said. If CTL responses were tuned exactly right and the cells recognised the viruses circulating in the body, then CTL suppression could be both efficient and permanent. An efficient CTL response would, in effect, work like a maximally-suppressive cock
