The investigators will compare the immunogenicity of influenza vaccine in adults and children with inflammatory bowel disease by timing of vaccine in relation to maintenance infliximab dosing. The primary objective is to compare the proportion of IBD patients on maintenance infliximab who mount serologic protection to each component of the influenza vaccination between patients vaccinated on day of infliximab infusion (Day 0 to 4) and patients vaccinated at the mid-point between infliximab infusions (Day 21 to 28). Serologic protection will be defined by a hemagglutination-inhibition titer ≥ 1:40. The secondary objective is to compare the proportion of IBD patients who mount an immunogenic response (≥ 4-fold increase from pre to post-vaccination titer) to each component of the influenza vaccination between patients vaccinated on day of infliximab infusion and those vaccinated at the mid-point.