The current study is a noninterventional prospective study examining the efficacy of additional dosage of the coronavirus disease 2019 (COVID-19) vaccine in kidney transplant recipients (KTRs).

In this clinical, prospective, non-interventional study to assess the effectiveness of an additional dose of COVID-19 vaccination, we evaluated the seroconversion rate of KTRs after the booster shot. Consecutive individuals who have completed standard two-doses of COVID-19 vaccination were enrolled. Following consent, demographic and clinical data were collected from participants, including patient/ transplant characteristics and information regarding previous COVID-19 vaccination.

1 Study visit and sample collection Patients were invited to return for up to 2 follow-up visits for sample collection. Whole blood samples were collected two weeks to 1 day before the booster vaccination and 4 weeks after. During the visit for the second blood sample, a survey was conducted to assess booster-related adverse events.

2. Anti-SARS-CoV-2 antibody assay IgG antibodies against the receptor-binding domain (RBD) of the S1 subunit of the spike protein of severe acute respiratory syndrome coronavirus 2(SARS-CoV-2) were quantified with SARS-CoV-2 IgG II Quant assay (Abbott).

Additionally, neutralizing antibody was assessed using anti-SARS-CoV-2 GenScript cPass SARS-CoV-2 Neutralization Antibody Detection Kit. The cPass detection kit utilizes the horseradish peroxidase (HRP) conjugated recombinant SARS-CoV-2 RBD protein and the human angiotensin-converting enzyme 2 (ACE2) receptor protein. It detects neutralizing antibodies capable of blocking protein interaction between HRP-RBD and ACE2. The assay was performed according to the manufacturer's instructions.

Inclusion Criteria:

1. Patients who received kidney-only transplantation or multi-organ transplantation including kidney before December 31, 2020
2. Patients ≥ 12 years old
3. 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.
4. Patients with plans of getting an additional dose of COVID-19 vaccine (i.e., booster shot)
5. Patients providing informed consent and willing to comply with the study protocol, including two blood tests and a survey.

Exclusion Criteria:

1. Patients with a previous history of SARS-CoV-2 infection
2. Who had B-cell depleting therapy (e.g., rituximab, bortezomib) or T-cell depleting therapy (anti-thymocyte globulin) within six months
3. Patients who restarted dialysis due to graft failure

Kidney transplant recipients fully vaccinated with standard dose(s) of COVID-19 vaccination