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The disappointing results from the HIV Vaccine Trials Network (HVTN) 502 phase IIB vaccine trial (also know as the STEP trial) has caused the scientific community to pause and wonder ‘What went wrong?’ and even worse ‘Is a vaccine against HIV possible?’ This trial demonstrated no potential benefit of the Merck Adenovirus 5 (Ad5) HIV vaccine, and in some participants with pre-existing immunity to Ad5, increased susceptibility to HIV infection. 1 Some of the answers to the first question will come from a full analysis of this trial as well as the South African equivalent trial (HVTN 503/Phambili) that was halted last year. Attempting to answer the second question has brought about a re-think of current paradigms. This has resulted in a call for a back-tobasics approach, requiring a better understanding of both the virus and human immune responses to HIV. History of vaccination The history of vaccination is a goodnews story—one from which we need to draw encouragement and inspiration in the wake of the failed Merck Ad5 vaccine. Smallpox was eradicated by vaccination with vaccinia, a virus related to smallpox; and polio should be eradicated in the near future. We have successful vaccines against measles, hepatitis A and B and many other viral infections—we know that vaccines work and in many cases we have a good idea of how they work. Part of the difficulty in making a vaccine against HIV is that we do not know the type of immune responses to stimulate in order to provide protection against either HIV infection or to control viremia. This is because unlike other viral infections, there is no spontaneous clearance of HIV from the body: once infection is established a person is infected for life. Thus, an HIV vaccine needs to do better than nature, which is no small task. Nevertheless, the immune response against HIV is extremely potent at keeping the virus in check for many years, although as a result of its remarkable ability to mutate, HIV evades and erodes these defences, eventually causing complete immune collapse. This exceptional variability, too, presents an enormous obstacle to vaccine developers, who are finding it difficult to design a vaccine which will protect against such enormous diversity.

Where are we going with HIV vaccines?