Studies in mice have supported the notion that agonists of pattern recognition receptors (PRRs) are promising vaccine adjuvant candidates. One such candidate is a type of synthetic double-stranded RNA (dsRNA) — polyinosinic–polycytidylic acid stabilized with poly-L-lysine (polyICLC) — which is an agonist of Toll-like receptor 3 (TLR3) and the cytosolic RNA helicase MDA5. But is polyICLC effective in humans? This study shows that subcutaneous injection of healthy volunteers with a single dose of polyICLC was tolerated, although local and systemic reactogenicity was noted. A systems-wide analysis of innate immune responses in these volunteers showed that polyICLC reliably induces an interferon response, and also induces other components of the main effector pathways of innate immunity, such as the inflammasome, complement and nuclear factor-κB pathways. The pathways induced by polyICLC were similarly induced by the highly efficacious yellow fever vaccine, so synthetic dsRNA can be considered a true viral mimic for use as a vaccine adjuvant.