Comparing different routes of inoculation with the vaccine strain M. bovis bacille Calmette-Guérin in mice shows that the frequency of IFN-γ-secreting CD4 T cells correlates more closely with disease severity than with protection from TB disease after M. tuberculosis challenge [8], and human trials with the vaccine candidate MVA85A showed that although this vaccine elicits significant numbers of IFN-γ-secreting CD4 T cells it does not lead to enhanced protection against infection [9,10]. Here, CD4 is linked to infection.