Sir

In your Editorial 'Broken promises' (Nature 452, 503; 2008), you say: “Decisions to move Merck's vaccine candidate and a previous failed candidate into clinical trials were based only partly on science.” I would like to clarify the position on the Merck vaccine trial, as there is no evidence to support this assertion.

The scientific consensus was that the phase IIb efficacy trial of this vaccine candidate was justified. Inducing CD8 T-cell responses in the majority of vaccine recipients by strategies other than live attenuated vaccines has proved extremely difficult, and the adenovirus serotype-5 platform was the first to overcome the problem.

The trial was designed to be smaller than a traditional phase III and included an interim analysis after the first 30 infections to ensure participant safety. It was this analysis by the Data Safety Monitoring Board that sparked the concern about safety with the adenovirus vector. The trial has certainly raised questions about whether the induction of HIV-specific T-cell responses is a viable vaccine strategy, but these questions are best addressed by analysing the data.

Particularly important are questions regarding the effect of the quality, quantity and specificity of the vaccine-induced CD8 T-cell response on post-infection viral load control. These questions could not be addressed without a vaccine approach that actually induced CD8 T-cell responses in most recipients.

The Merck vaccine was the first such candidate, so it is a misleading exaggeration to claim that its failure is a “crisis” for HIV vaccine research. A journal with Nature's long history is well placed to know how likely first-time successes are in science.

In my view, your Editorial risks reinforcing the unrealistic expectations of science that you erroneously imply were promoted by the HIV-vaccine researchers involved in the Merck trial.