CD4 and influenza: In a randomized, observer-blind, placebo-controlled trial study in elderly (≥65 y) volunteers without unstable medical conditions treated with EVR, an enhanced response to influenza vaccination by about 20% was shown.21 The proposed mechanism was the decrease in programmed death-1 (PD-1)–positive CD4+ and CD8+ T cells found in EVR-treated patients, compared with placebo-treated patients.