As solid organ recipients are at high risk of severe COVID-19 and respond poorly to primary SARS-CoV-2 mRNA vaccination, they have been prioritized for booster vaccination. However, an immunological correlate of protection has not been identified in this vulnerable population. We conducted a prospective monocentric cohort study of 65 kidney transplant recipients who received three doses of SARS-CoV-2 BNT162b2 mRNA vaccination. Associations between symptomatic breakthrough infection (BTI) and vaccine responses, patient demographic and clinical characteristics were explored. Symptomatic COVID-19 was diagnosed in 32% of kidney transplant recipients during a period of six months after the administration of the third vaccine dose. During this period, SARS-CoV-2 delta and omicron were the dominant variants in the general population. Univariate analyzes identified avidity of SARS-CoV-2 receptor binding domain (RBD) binding IgG, neutralizing antibodies and SARS-CoV-2 S2 domain-specific IFN-gamma responses as correlates of protection against BTI. Some demographic and clinical parameters correlated with vaccine responses, but none correlated with the risk of BTI. In multivariate analysis, the risk of BTI was best predicted by neutralizing antibody and S2-specific IFN-gamma responses, adjusting for age, graft function and mycophenolate mofetil use. In conclusion, both antibody and T cell responses predict the risk of BTI in kidney transplant recipients who received three doses of SARS-CoV-2 mRNA vaccine. T cell responses may help compensate for the suboptimal antibody response to vaccination in this vulnerable population.

The authors have declared no competing interest.

This study was co-funded by the Belgian Federal Government through Sciensano and by the Fonds de la Recherche Scientifique, F.R.S.-FNRS, Belgium.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethics committee of the Erasme Hospital (P2020/284 and A2021/131) and the Belgian Federal Agency for Medicines and Health Products (FAMHP, EudraCT 2021-000-412-28) gave ethical approval for this work.

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All data produced in the present study are available upon reasonable request to the authors