Influenza infection occurring during oncologic treatment or following hematopoietic cell transplantation (HCT) is associated with increased risk of morbidity in the form of lower respiratory tract infection (LRTI) and mortality relative to otherwise healthy patients. The study participants have been diagnosed with a hematological malignancy and are eligible to receive the current seasonal influenza (Flu) vaccine.

Primary Objective

• To determine the feasibility of opening a longitudinal prospective study of IIV immunogenicity in pediatric leukemia patients.
• To describe the immunogenicity, as measured by the development of cell- and/or antibody-mediated influenza specific responses 3 to 5 weeks following vaccination, in a cohort of pediatric leukemia patients.

Secondary Objectives

• To describe whether an immune response, as measured by development of cell- and/or antibody-mediated influenza specific responses, is detectable 1-2 weeks following vaccination in a cohort of pediatric leukemia patients.
• To describe the durability of immunogenicity by measuring cell - and antibody- mediated influenza specific responses at 6 months and 1 year following vaccination in a cohort of pediatric leukemia patients.

Exploratory Objectives

• To estimate the clinical effectiveness of influenza vaccine in this cohort by monitoring for the development of clinical diagnosis of influenza in the cohort of enrolled pediatric oncology patients.
• To correlate results of immune cell frequency in blood, as measured by complete blood count with differential, with development of an immune response to IIV.

This is a prospective, 2-stage, non-therapeutic study to characterize the immunogenicity of influenza vaccination among pediatric oncology patients at St. Jude. The first stage of the study will consist of a feasibility phase to assess the feasibility of opening a longitudinal prospective study of IIV immunogenicity in pediatric leukemia patients. The second stage will assess the immunogenicity of IIV among pediatric leukemia patients in a larger samples size. The investigator will collect about one teaspoon of blood from the study participant up to 6 times over a year for a total of 6 teaspoons a year. The investigator will also collect some information about the study participant's underlying condition, flu testing and overall health from the medical record during this time.

Inclusion Criteria:

• Patient ≤ 18 years old at the time of consent receiving care at St. Jude Children's Research Hospital
• Diagnosed with a hematological malignancy
• Eligible to receive the current seasonal influenza vaccine

Exclusion Criteria:

• Previously received at least one dose of the current seasonal influenza vaccine.

All participants who meet eligible criteria and consent to enrollment on the study.