Humoral Immune Response to a Timely Booster mRNA Vaccination in Non-Responders to a Standard Vaccination Schedule against COVID-19 in Kidney Transplant Recipients.

Kidney transplant recipients who are at increased risk for COVID-19 infection and associated morbidity and mortality have been shown to be prone to an impaired humoral immune response to a standard vaccination schedule against COVID-19 with two doses of SARS-CoV-2 mRNA vaccines. In this study, response rate of 94 kidney transplant recipients without detectable seroconversion after two doses of a mRNA vaccine who were offered a timely third mRNA vaccine after completion of the standard vaccination schedule was retrospectively analyzed. After a median of 28 days, antibody titers against the S1 spike protein showed a non-response rate of 53%. No significant risk factors for non-response could be identified. The responders showed a high variation in antibody titers (median 73.9 BAU/mL, IQR 221.5). In conclusion, a third booster mRNA vaccine in non-responding kidney transplant recipients leads to a detectable humoral immune response in approximately half of the patients. In the seroconversion group, antibody titers were highly variable, indicating that even non-responders to the standard vaccination schedule might develop a significant humoral immune response after a timely booster vaccine.