Abstract
Background
Solid organ transplant recipients taking immunosuppressive drugs are at greater risk of severe COVID-19 than the general population. In particular, kidney transplant recipients (KTRs) are known to have lower seropositivity after basal doses of SARS-CoV-2 vaccines, and the strategy of administering booster doses in these immunocompromised individuals has been promoted worldwide.
Methods
This study evaluated the effect of a fourth dose (D4) of SARS-CoV-2 vaccine in Japanese KTRs. Anti-spike (anti-S) IgG antibody titers at 1 and 3 months after D4 of SARS-CoV-2 vaccine were evaluated in 75 KTRs.
Results
The median anti-S IgG antibody titers at 1 and 3 months after D4 were 4728.1 (interquartile range [IQR]: 643.2–13,953.1) AU/mL and 3778 (IQR: 642–9436.6) AU/mL, respectively. The seropositivity rate after D4 was 85.1% at 1 month and 83.1% at 3 months, and the seroconversion rate was 28.6% (4 of 14 KTRs seronegative after the third dose). Factors associated with poor humoral responses were shorter time post-transplant to infection, a higher mycophenolate mofetil dose, a lower lymphocyte count, and a lower estimated glomerular filtration rate.
Conclusion
This study demonstrates some efficacy of D4 of SARS-CoV-2 vaccine in KTRs who are seronegative after three doses and encourages consideration of further booster doses of the SARS-CoV-2 vaccine.




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The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.
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Acknowledgements
This research received no specific grant from funding agencies in the public, commercial, or non-profit sectors. We thank Teruko Tamatsukuri, University School of Medicine, Tokyo, Japan, for her expert assistance.
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A.H., M.K., I.Y., and Hiroyasu Yamamoto participated in the clinical practice, designed the study protocol, and drafted the manuscript. Y.O., A.K., N.M., Y.T., T.H., and I.O. participated in patient care and revised the manuscript. F.U., J.M., Hiroki Yamada, and T.K. performed kidney transplantation. T.Y. supervised all authors as division director. All authors participated in the preparation of the manuscript and approved the final version.
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Hayashi, A., Kawabe, M., Yamamoto, I. et al. Booster effect of the fourth dose of the SARS-CoV-2 mRNA vaccine in kidney transplant recipients. Clin Exp Nephrol (2025). https://doi.org/10.1007/s10157-025-02651-6
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DOI: https://doi.org/10.1007/s10157-025-02651-6