*Purpose: Evaluate antibody (Ab) response after catch-up vaccinations in pediatric kidney transplant candidates (PKTC).

*Methods: A single center retrospective review of PKTC less than or equal to 18 years old who received catch-up vaccinations from 2/2017 to 11/2019 was conducted. Anti-HLA Ab were detected at 3-4 and 7-10 weeks post catch-up vaccinations by single antigen assay. De novo anti-HLA Ab was defined as a change from negative mean fluorescence intensity (MFI) to positive MFI. Positive MFI strength categories include weak: 1000 – 2999; moderate: 3000 – 9999; strong: greater than 10,000.

*Results: Sixty-four vaccinations were administered to 14 patients. Forty-one potential de novo anti-HLA Ab were detected in 5 patients, all without history of sensitizing events from time of evaluation to follow-up. After additional Ab panel testing, 5/41 potential de novo anti-HLA Ab were determined to be HLA specific; the remaining 36 were deemed non-specific. Median MFI increase was significant for non-specific, but not de novo anti-HLA Ab (Table 1). Median MFI values peaked at 3-4 week but diminished by 7-10 week follow-up (Figure 1). No HLA-donor specific Ab developed from the time of transplant to the end of follow-up in the 13 PKTC that continued on to transplant. One patient experienced antibody mediated rejection that did not correlate to any pre-formed or de novo anti-HLA-Ab that emerged post vaccinations.

*Conclusions: Vaccination resulted in a significant, and transient, increase in median MFI from baseline to 3-4 week follow-up that decreased by 7-10 week follow-up. The majority of responses were non-specific, hypothesized to be related to denatured antigens on single antigen beads. Five confirmed de novo anti-HLA Ab were detected and deemed secondary to vaccination. These data suggest limited clinical impact of vaccinations on emergence of de novo anti-HLA Ab.

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