High vs.Standard Dose Influenza Vaccine in Pediatric Solid Organ Transplant (SOT) Recipients

Study design: This is a phase II, multi-center, double-blind, randomized controlled immunogenicity and safety trial comparing two doses of HD-QIV or two doses of SD-QIV in pediatric SOT recipients. Hypotheses: 1. Pediatric SOT recipients who are 1-23 months out from transplant and are administered two doses of HD-QIV will develop higher Hemagglutination Inhibition (HAI) geometric mean titer (GMT) to influenza antigens compared to pediatric SOT recipients receiving two doses of SD-QIV, with Geometric Mean Titer Ratio (GMR) HD-QIV/SD-QIV greater than 1.0. 2. Administration of HD-QIV in pediatric SOT recipients will be well tolerated and the safety profile will be similar to SD-QIV with regards to solicited local and systemic post-administration reactions. 3. Baseline immunophenotypic markers of exhaustion, immune senescence, and immune activation at the pre-vaccine timepoint will correlate with post-vaccine HAI titers. Study population: The study plans to enroll a total of approximately 312 pediatric heart, liver, and/or kidney transplant recipients between 1 and 23 months post-transplantation. Study enrollment: The enrollment period will be over three-years. Participants will be randomized into one of two groups. Group 1 will receive two doses of SD-QIV (0.5 mL; 15μg of each influenza antigen) whereas Group 2 will receive two doses HD-QIV (0.7 mL; 60μg of each influenza antigen). Influenza surveillance: Active surveillance for influenza-like symptoms will begin when influenza season starts in each site's community, defined in previous trials as identification of at least two positive respiratory tests for influenza, with at least 10% of diagnostic tests positive during two consecutive weeks in the local clinical or research laboratory. Enrollment will continue during influenza season with nasal swabs obtained at all main visits to document the occurrence of influenza virus both prior to and after vaccination. During the influenza season, the study staff will attempt to do a weekly telephone and/or electronic communication with the participants to detect and document any influenza-like illness (ILI) and any specific coronavirus disease of 2019 (COVID-19) like symptoms. If participants meet ILI criteria and/or any specific coronavirus disease of 2019 (COVID-19) like symptoms (see below), an additional nasal swab will be collected\*. ILI criteria are met by occurrence of one of the conditions below: * Fever: ≥38°C (100.4°F) * Two or more of any of the following: respiratory symptoms (rhinorrhea, sinus congestion, post-nasal drip, shortness of breath, cough, wheezing, sputum production, sore throat, sneezing, watery eyes, ear pain, hoarseness); or systemic symptoms (myalgias, chills, chest pain, or headache); or new loss of taste or new loss of smell; or gastrointestinal symptoms (diarrhea or vomiting). * Per investigators' discretion at each individual site, a swab is not needed if there is a known non-respiratory cause of symptoms.