*Purpose: Even though a high efficacy and immunogenicity of COVID-19 vaccines have been reported in the general population, vaccine immunogenicity is suboptimal in SOT recipients and breakthrough SARS-CoV-2 infection has already been reported in this immunocompromised population. Thus, several approaches including booster dose administration was investigated and showed better outcome. However, as a booster, mix and match method has not been investigated enough yet. The aim and objectives of this study is to check the immunogenicity after third dose of the SARS-CoV-2 vaccines either with adenovirus vector vs. mRNA vaccine.

*Methods: This is a single center, single blinded (patient blinded), randomized controlled trial comparing BNT 162b2 and JNJ-78436735 as a third dose after completion of two doses of BNTT 162b2 vaccine in SOT recipients. We included adult SOT recipients with functional graft on at least one immunosuppressive medication. Also, the participants should have completed two doses of BNT162b2 vaccination at least 28 days prior to the third dose. As a primary end point, we are going to check the anti-spike protein of SARS-CoV-2 IgG positive rate in one month after vaccination.

*Results: We have finished the enrollment and there were 60 SOT recipients including 39 kidney, 11 liver, 4 lung, 2 heart and 4 combined organ transplant recipients. Already, 58 recipients completed the follow up blood test visit. Median age of the enrolled recipients was 56.7 (IQR 51 – 63) and 22 (37.9%) were female. As a maintenance immunosuppression, 52 (89.7%), 45 (77.5%) and 26 (44.8%) recipients received tacrolimus, mycophenolate and prednisone, respectively.

*Conclusions: Immunogenicity data will be analyzed once the follow up blood test result are finished. Adverse events and the rate of COVID-19 also will be reported.

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