PROJECT SUMMARY Influenza virus is a significant pathogen in pediatric solid organ transplant (SOT) recipients. However, these individuals respond poorly to standard-dose (SD) inactivated influenza vaccine (IIV). Recent studies have investigated two strategies to overcome poor immune responses in SOT recipients: (1) administration of high- dose (HD)-IIV compared to SD-IIV and (2) two doses of SD-IIV compared to one dose of SD-IIV in the same influenza season. One study compared HD-IIV vs. SD-IIV in adult SOT recipients and noted that HD-IIV was safe and more immunogenic; however, the median post-transplant period was 38 months. A phase I pediatric study comparing a single dose of HD-IIV vs. SD-IIV was safe with higher immunogenicity, but the study was limited by small sample size and median post-transplant vaccine administration was 26 months. In another phase II trial of adult SOT recipients, two doses of SD-IIV one month apart compared to one-dose of SD-IIV revealed modestly increased immunogenicity when given at a median of 18 months post-transplant. Therefore, these studies lack both evaluation in the early post-transplant period and substantive pediatric populations. Additionally, the administration of two-doses of HD-IIV in the same influenza season has not been evaluated in pediatric SOT recipients. Thus, the optimal immunization strategy for pediatric SOT recipients less than 24 months post-transplant is unknown. In addition, immunologic predictors and correlates of influenza vaccine immunogenicity in pediatric SOT recipients have not been well-defined. The central hypothesis of our proposal is that pediatric SOT recipients 1-23 months post-transplant who receive two doses of HD- quadrivalent inactivated influenza vaccine (QIV) will have similar safety but higher HAI geometric mean titers (GMTs) to influenza antigens compared to pediatric SOT recipients receiving two doses of SD- QIV. To test this hypothesis and address the critical knowledge gaps outlined above, we propose to conduct a phase II, multi-center, randomized-controlled immunogenicity and safety trial comparing two doses of HD-QIV to two doses of SD-QIV in pediatric kidney, heart, and/or liver SOT recipients 1-23 months post-transplant. The results of this study will address significant knowledge gaps regarding influenza vaccine strategies and immune responses in pediatric SOT recipients and will guide vaccine recommendations in the early post- transplant period.

NARRATIVE Influenza virus is a significant pathogen in pediatric solid organ transplant (SOT) recipients, but these individuals respond poorly to standard-dose (SD) inactivated influenza vaccine (IIV). Studies suggest that two doses of SD-IIV or one dose of high-dose (HD-IIV) is superior to one dose of SD-IIV; however, this has not been tested in the highest risk early post-transplantation period, and pediatric-specific studies of these regimens and their impact are critically lacking. This study aims to compare two doses of HD-IIV to two doses of SD-IIV administered in the same influenza season in pediatric kidney, heart, and/or liver SOT recipients in the early post-transplant period.