IL21 and COVID-19: Nevertheless, we found that the HD patients who display worsening of COVID-19 symptoms exhibited a large increase in serum cytokine levels after HD session (>2.5-fold increase for GM-CSF, IFN-γ, IL-1β, IL-2, IL-6, IL-17A and IL-21), which was not observed in uninfected HD patients or COVID-19 HD patients without clinical deterioration.