Introduction: Vaccination against SARS-CoV-2 reduces COVID-19 mortality and complications in solid organ transplant recipients. We evaluated the associated antibody responses and breakthrough infections following vaccination with inactivated SARS-CoV-2 vaccine (CoronaVac, Sinovac) or BNT162b2 mRNA vaccine (Comirnaty, Pfizer-BioNTech) in this high-risk population.

Method: This prospective observational study (April 2021-Febbruary 2022) included 10 liver and 38 kidney transplant recipients who received 2 vaccine doses (Sinovac, n = 31; or BioNTech, n = 17) as primary series all of whom provided blood samples (4-6 weeks after second dose) for quantitative antibody tests (Abbott Quant assay forimmunoglobulin G antibodies against SARS-CoV-2 spike protein). Following the end of the primary series, all patients were monitored for the development of infection for 9 months. The booster doses given to the patients throughout the follow-up were also documented. Type I error was α = .05 in all statistical analyses (SPSS, version 25).

Results: We analyzed demographic data, antibody responses after 2 doses of SARSCoV-2 vaccine as primary series, booster doses and development of breakthrough infection. Median age was 36.5 and 35 (73%) of them were male. Five patients developed COVID-19 in the first six months after the primary series were completed, four of them had received two doses of Sinovac and one had received two doses of BioNTech. Only one patient has needed hospitalization vaccinated with two doses of Sinovac and her antibody level was 0 AU/mL. The median antibody level in patients who developed COVID-19 was 41,9 AU/mL, while it was 312,0 AU/mL in those who did not. However the difference was not statistically significant (p=0,092). Between the 6th and 9th months, 11 participants tested positive for SARS CoV-2 PCR, with all isolates being Omicron variant. While the rate of positivity was 17% in patients who had at least two booster doses of mRNA vaccine after two consecutive doses of primary series, it was 27% in patients who received just one dose or no booster.

Conclusion: Solid-organ transplant recipients demonstrated inadequate vaccine responses. In addition, with the decrease in the immune response developed with the vaccine over time, it is seen that the newly emerged variants significantly increase the need for additional doses. In solid organ transplant patients, the CDC recommends a primary series of three doses at one-month intervals with a booster dose three months later. Despite the small amount of data, the findings of our study support this requirement.