Inclusion Criteria:  Solid organ transplant patients in the last 10 years and currently under immunosuppressive therapy Vaccination with two doses of Coronavac vaccine or BNT162b2 vaccines, followed by a booster dose (3d dose) of BNT162b2 vaccine administered in the previous 8-12 weeks.  Exclusion Criteria:  Previous SARS-CoV-2 infection Booster vaccine (3rd dose) administered less than 8 weeks or more than 12 weeks before enrolment Intravenous immunoglobulin therapy 60 days before enrolment Previous SARS-CoV-2 vaccine different from CoronaVac or BNT162b2