IL15 and COVID-19: Among patients with symptomatic COVID-19 (N = 240), hierarchical correlation matrices and force-directed network models identified two groups of strongly correlated mediators centered around IFN-α2/IFN-γ and TNF/lymphotoxin-α, respectively, with IL-15 appearing as a key inter-group mediator (Fig. 5a, b and Supplementary Fig. 5).