This is an open-label, single-arm clinical trial designed to evaluate the immunogenicity and safety of a booster dose of an adjuvanted recombinant SARS-CoV-2 spike protein subunit vaccine (SpikoGen) produced by CinnaGen Co. in kidney transplant recipients after two doses of Sinopharm's inactivated virus vaccine. A total of 100 adult individuals receive a single dose of the SpikoGen COVID-19 vaccine at 1 to 3 months after the second dose of the Sinopharm COVID-19 vaccine. The injection is given in the deltoid muscle of the non-dominant arm. For immunogenicity assessments, blood samples will be collected one month after the booster injection. For safety assessments, all participants will be followed up for one month.

Study hypotheses include:

1. A booster dose of the SpikoGen COVID-19 vaccine induces strong immunogenicity against SARS-CoV-2 in adult kidney transplant recipients who were fully vaccinated with Sinopharm COVID-19 vaccine.
2. A booster dose of the SpikoGen COVID-19 vaccine is safe and tolerable in adult kidney transplant recipients who were fully vaccinated with Sinopharm COVID-19 vaccine.

Inclusion Criteria:

• Male or female ≥18 years
• Willing and able to comply with all study requirements, including scheduled visits, intervention, and laboratory tests
• Kidney transplant recipients who had received two doses of Sinopharm vaccine after transplantation
• Females must not be pregnant or breastfeeding
• At least six months should have passed from the time of transplantation
• Between 1 to 3 months should have passed from the second dose of Sinopharm vaccine

Exclusion Criteria:

• Subjects with signs of active SARS-CoV-2 infection at the screening visit
• Subjects with a history of SARS-CoV-2 infection based on a positive PCR test result after the second dose of the primary vaccination
• Subjects with an active CMV infection that requires treatment
• Subjects who have received rituximab within 6 months prior to the screening visit
• Subjects who have received intravenous immune globulin (IVIG) within 6 months prior to the screening visit
• Subjects who have a history of severe allergic reactions (e.g., anaphylaxis) to the study vaccine, any components of the study interventions, or any pharmaceutical products.
• Subjects who have received any other investigational products within 30 days prior to the screening visit or intend to participate in any other clinical studies during the period of this study.
• Subjects who have experienced transplant rejection within 30 days prior to the screening visit
• Subjects with any condition that may increase the risk of participating in the study or may interfere with the evaluation of the primary endpoints of the study in the investigator's opinion.