A Study to Evaluate the Safety and Efficacy of Upadacitinib in Participants With Giant Cell Arteritis

Inclusion Criteria: * Diagnosis of giant cell arteritis (GCA) according to the following criteria: * History of erythrocyte sedimentation rate (ESR) \>= 50 mm/hour or high sensitivity C-reactive protein (hsCRP)/CRP \>=1.0 mg/dL * Presence of at least one of the following: Unequivocal cranial symptoms of GCA or Unequivocal symptoms of polymyalgia rheumatica (PMR) * Presence of at least one of the following: temporal artery biopsy revealing features of GCA or evidence of large vessel vasculitis by angiography or cross-sectional imaging such as ultrasound, magnetic resonance imaging (MRI), computed tomography (CT) or positron emission tomography (PET). * Active GCA, either new onset or relapsing, within 8 weeks of Baseline. * Participants must have received treatment with \>=40 mg prednisone (or equivalent) at any time prior to Baseline and be receiving prednisone (or equivalent) \>= 20 mg once daily (QD) at Baseline. * Participants must have GCA that, in the opinion of the investigator, is clinically stable to allow the participant to safely initiate the protocol-defined corticosteroid (CS) taper regimen. * Females must either be postmenopausal or permanently surgically sterile or, practicing at least 1 specified method of birth control through the study. Exclusion Criteria: * Prior exposure to any Janus Kinase (JAK) inhibitor. * Treatment with an interleukin-6 (IL-6) inhibitor within 4 weeks of study start, or prior treatment with an IL-6 inhibitor and experienced a …