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When Merck & Co Inc.'s V520 HIV vaccine failed in last year's Phase II STEP trial, it was the most advanced T cell-based HIV vaccine in development. Two research teams now have posited different theories for the failure. Whether adenovirus-induced immunogenicity is to blame, as one paper suggests, or heterologous prime-boost regimes will be needed, as the other suggests, the findings offer some hope for future development of HIV and T cell-based vaccines.

V520 was designed to induce cytotoxic T cells capable of killing HIV-infected T cells. The vaccine consisted of an adenovirus serotype 5 (Ad5) vector encoding three HIV proteins-gag, pol and nef-and was designed to be used for both primary and booster shots...