Vaccine Response After Rituximab for Chronic, Severe Idiopathic Thrombocytopenic Purpura

The purpose of this ancillary, pilot-phase study is to determine how children with a history of severe, chronic ITP who were treated with rituximab might respond to vaccines. Eligible patients are previously or currently enrolled in a study entitled "Open Label, Phase I/II Trial of Rituximab for Chronic, Severe Idiopathic Thrombocytopenic Purpura in Children and Adolescents" (CHB 02-12-160) and have decided to obtain the trivalent, inactivated influenza vaccination. These patients will be invited to provide one blood sample prior to vaccination and a second sample 4-8 weeks after vaccination to quantify immune response to vaccination. Additionally, if patients are scheduled to receive a tetanus booster vaccination within one month before or after the influenza vaccination, response to tetanus will also be quantified. This sample will be collected during the same phlebotomy as the influenza sample. In some cases, a blood sample was stored prior to rituximab treatment and will be used for baseline assessment. The primary and secondary objectives for this study are as follows: Primary: * To determine the portion of patients who will respond adequately to influenza vaccination, with adequacy defined as a titer greater than 1:32 for each strain of virus in the vaccine, measured 4-8 weeks after administration of the vaccine OR greater than a four-fold increase in titers measured 4-8 weeks after administration of the vaccine Secondary Objectives: * To evaluate the ability to mount a response to the influenza vaccine, with response defined as any increase in influenza antibody titer for each strain of virus between samples before and 4-8 weeks after vaccination. * To evaluate the ability to mount an adequate response to tetanus toxoid, with adequacy defined as in the primary objective. * To evaluate the ability to mount a response to tetanus toxoid, with response defined as above. * To compare response to influenza vaccination received less than one year after rituximab and greater than one year after rituximab.