Booster effect of the fourth dose of the SARS-CoV-2 mRNA vaccine in kidney transplant recipients

BackgroundSolid 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.MethodsThis 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.ResultsThe 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.