Humoral response after a third and fourth dose of mRNA-based SARS-CoV-2 vaccine in previously seronegative kidney transplant recipients.

Growing evidence shows diminished response to mRNA-based SARS-CoV­2 vaccination in kidney transplant recipients. We aimed to investigate the seroconversion rate after a 3rd and 4th dose of mRNA vaccination in kidney transplant recipients without prior antibody response to two or three vaccination doses.This retrospective study included 324 prevalent kidney transplant recipients of a single tertiary transplantation center of which 157 remained seronegative, defined as anti-spike-RBD-IgG antibody titer < 7.1 BAU/ml, after two doses of mRNA-based SARS-CoV­2 vaccination. Maintenance immunosuppression was not changed. The median patient age was 60.6 years (IQR 51.4-68.1 years), 66.9% were male. Positivity for anti-spike-RBD-IgG (≥ 7.1 BAU/ml) was measured 4-5 weeks after administration of a 3rd and 4th vaccine dose.Seroconversion rates were 63.9% after a 3rd dose and 29.3% after a 4th dose of vaccine. Cumulative prevalence of seropositivity was 51.5% after 2 doses, 80.5% after 3 doses and 84.2% after 4 doses.In conclusion, seroconversion can be achieved in the majority of the kidney transplant recipients by administrating three or four doses of mRNA vaccine without changing maintenance immunosuppression.