Inclusion Criteria:  Age > 18 years; Male or female sex; Renal transplant with stable renal function in the last 2 months prior to study inclusion. Had received a full schedule of vaccination for SARS-CoV-2 with mRNA1273 vaccine (Moderna) and remained seronegative at 2 weeks after the second dose. Patient giving written informed consent.  Exclusion Criteria:  Renal transplantation < 3 months; Pregnancy or lactation status; Rejection treated within the last 6 months; Presence of humoral immunity to SARS-CoV-2 defined as the presence of specific IgM/IgG.