CD4 and malaria: QS-21 is a saponin fraction extracted from Quillaja saponaria Molina.2 AS01 is included in the recently developed malaria vaccine RTS,S (Mosquirix)3 and in other candidate vaccines in development against herpes zoster (HZ/su),4 HIV,5 and tuberculosis.6 The promotion of antigen-specific CD4+ T cells in addition to antigen-specific antibodies sets AS01 apart from other adjuvants.2 AS01-adjuvanted vaccines have also been effective in challenging populations, such as infants (with RTS,S against malaria3) and the older adults (HZ/su against herpes zoster4).