IL21 and COVID-19: The higher activated status of and IL-21 secretion by CXCR3+ TFH cells compared with CXCR3− TFH cells upon antigen exposure indicates that spike-specific CXCR3+ TFH cells may play a greater role in supporting B cell function than spike-specific CXCR3− TFH cells in both COVID-19 convalescents and vaccinees.