*Purpose: Prior studies demonstrated an increased immune response post-3^rd COVID-19 vaccine dose when given 1-month post-2^nd dose in solid organ transplant recipients (SOT). This study assessed whether a 3^rd mRNA vaccine dose administered 6 months post-2^nd dose enhanced humoral immune response in SOT.

*Methods: A prospective cohort study was conducted of SOT, without prior COVID-19, who received 3 mRNA vaccine doses (BNT162b2 (Pfizer) or mRNA-1273 (Moderna)). Primary outcome was a positive serologic response characterized by an anti-receptor-binding domain (RBD) antibody (Ab) level of >100 U/mL 4 weeks post-3rd dose (measured with the Roche Elecsys anti-SARS-CoV-2 immunoassay).

*Results: 175 SOT were enrolled: 48 (27%) heart, 72 (41%) lung, 51 (29%) kidney, 4 (2%) other SOT (Figure 1). A positive anti-RBD Ab level measured 4-weeks post-3^rd vaccine dose was present in 105/175 (60%). In a multivariable model, age >60 years (adjusted odds ratio [aOR] 0.41; 95% CI 0.19 to 0.87), heart (aOR 0.28; 95% CI 0.10 to 0.76) or lung (aOR 0.20; 95% CI 0.07 to 0.55) transplant, and mycophenolate use (aOR 0.24; 95% CI 0.11 to 0.54) were independent risk factors for not developing a positive Ab post-3^rd dose. Patients transplanted >10 years ago were more likely to have a positive Ab (aOR 4.95; 95% CI 1.04 to 23.42). Those with a low positive Ab (>0.8 to <100 U/mL) post-2^nd vaccine dose were more likely to have a positive Ab post-3^rd dose than those with an undetectable Ab (<0.8 U/mL). But 26/105 (25%) participants with an undetectable Ab post-2nd dose developed an Ab of >100 U/mL; 9/26 (35%) developed an Ab >1000 U/mL (Figure 2). 4 (2%) SOT developed rejection within 30-days post-3^rd dose.

*Conclusions: Despite the extended time interval of 6 months between the 2^nd and 3^rd doses, the proportion of non-responders (40%) in this study was similar to other studies when the 3^rd dose was given 1-month post-2^nd dose. These data suggest that a substantial proportion of SOT remain at risk for COVID-19 after 3 mRNA vaccine doses. Additional interventions need to be further studied, including monoclonal Ab as pre-exposure prophylaxis and 4^th vaccine doses.

« Back to 2022 American Transplant Congress