Primary varicella infection in children with systemic juvenile idiopathic arthritis under tocilizumab therapy.

We report the clinical course and outcome of primary varicella infection in six children with systemic juvenile idiopathic arthritis (sJIA) receiving tocilizumab. None had disseminated or fatal varicella infection, but one patient developed macrophage activation syndrome (MAS) and another had an arthritis relapse. All patients had a significant elevation of serum IL-6 levels, and the two children who developed MAS or arthritis relapse showed high serum IL-18 levels, which could cause a sJIA flare-up.