Using AI we identified baricitinib as possessing anti-viral and anti-cytokine efficacy. We now show a 71% (95% CI 0.15-0.58) mortality benefit in 83 patients with moderate-severe SARS-CoV-2 pneumonia with few drug-induced adverse events, including a large elderly cohort (median age 81 years). A further 48 cases with mild-moderate pneumonia recovered uneventfully. Using organotypic 3D cultures of primary human liver cells, we demonstrate that interferon-alpha-2 (IFNα2) significantly increases ACE2 expression and SARS-CoV-2 infectivity in parenchymal cells by >5-fold. RNA-Seq reveals gene response signatures associated with platelet activation, fully inhibited by baricitinib. Using viral load quantifications and super-resolution microscopy, baricitinib exerts activity rapidly through the inhibition of host proteins (numb associated kinases), uniquely amongst anti-virals. This reveals mechanistic actions of a Janus kinase-1/2 inhibitor targeting viral entry, replication and the cytokine storm, and is associated with beneficial outcomes including in severely ill elderly patients, data that incentivizes further randomized controlled trials.