Inclusion Criteria:  Patients who received kidney-only transplantation or multi-organ transplantation including kidney before December 31, 2020 Patients ≥ 12 years old Patients fully vaccinated with two standard doses of COVID-19 vaccination with the vector vaccine (AZD1222 of AstraZeneca) or the mRNA vaccines (BNT162b2 of Pfizer-BioNTech or mRNA-1273 of Moderna) including homologous and heterologous regimen. Patients with plans of getting an additional dose of COVID-19 vaccine (i.e., booster shot) Patients providing informed consent and willing to comply with the study protocol, including two blood tests and a survey.  Exclusion Criteria:  Patients with a previous history of SARS-CoV-2 infection Who had B-cell depleting therapy (e.g., rituximab, bortezomib) or T-cell depleting therapy (anti-thymocyte globulin) within six months Patients who restarted dialysis due to graft failure