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

Summary Growing evidence shows diminished response to mRNA-based SARS-CoV‑2 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-CoV‑2 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.