*Purpose: Evolving data suggests booster vaccine doses enhance the immunogenicity of SARS-CoV-2 vaccines in solid organ transplant recipients with higher IgG responses, neutralizing antibodies titers, and greater SARS-CoV-2-specific T-cell counts. Currently, there is no recommended framework for monitoring potential vaccine-related immunological graft injury. Here, we describe kinetics of dd-cfDNA pre- and post-booster vaccination in kidney transplant recipients (KTRs).

*Methods: Electronic medical records were reviewed to identify KTRs that received a SARS-CoV-2 booster vaccine dose in 2021 and were monitored with dd-cfDNA pre- and post-vaccination. dd-cfDNA was collected as part of standard of care assessment. Pre-booster dd-cfDNA levels were defined as the most recent result prior to booster administration. Post-vaccination results were collected up to 30 days post-booster administration.

*Results: 116 KTRs were identified for analysis. Patient demographics are summarized in Table 1. Median time from transplant to SARS-CoV-2 booster administration was 463 days (IQR 333-787.25, Table 1). Pre-booster dd-cfDNA levels were established a median of 9 days (IQR 2.25 – 16) pre-booster. The median level of dd-cfDNA pre-booster was 0.17% (IQR 0.12% – 0.25%). There was no significant difference in median levels of dd-cfDNA up to 30 days post-booster administration (Kruskal Wallis test with multiple comparisons, all p values >0.99, Figure 1). No adverse clinical events or acute rejection episodes were reported within 30 days of SARS-CoV-2 booster administration in this cohort.

*Conclusions: Median dd-cfDNA levels were not impacted by SARS-CoV-2 booster administration, suggesting that patterns of subclinical injury that may potentiate inflammation, allosensitization or allograft rejection are unlikely in this setting. The stability of dd-cfDNA demonstrated here further reinforces the safety profile of SARS-CoV-2 vaccine booster administration in KTRs.

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