Supplementary Material for: Humoral Response to the Third Dose of BNT162b2 COVID-19 Vaccine among Hemodialysis Patients

<b><i>Background:</i></b> Hemodialysis patients are at high risk for severe COVID-19 disease. Despite a high early seropositivity rate, dialysis patients mount a dampened immune response following two doses of an mRNA vaccine. This study aimed to evaluate the serologic response to a booster dose of BNT162b2 vaccine, 6 months after the second dose, among hemodialysis patients. <b><i>Methods:</i></b> This prospective study included 80 hemodialysis patients and 56 healthcare workers serving as controls. Serologic samples were evaluated before and ∼3 weeks after the third vaccine dose. The primary outcomes were the seropositivity rate and the log-transformed anti-SARS-COV-2 S1 (RBD) IgG as a continuous variable after the third dose. Secondary outcomes were the proportion of participants with “high response,” defined as antibody levels &gt;1,000 AU/mL, and “robust response,” defined as antibody levels &gt;4,160 AU/mL, according to prespecified cutoff values associated with neutralizing antibodies. Univariate and multivariate analyses were conducted to identify predictors of antibody response. <b><i>Results:</i></b> Among 80 hemodialysis patients, seropositivity rates improved from 78% (62/80) before the third dose, up to 96% (77/80) after the booster dose. The S1-RBD log-transformed antibody level increased significantly following the third dose from 2.15 ± 0.75 to 3.99 ± 0.83 compared with 2.65 ± 0.4 to 4.31 ± 0.42 in the control group. Among the hemodialysis patients, 88% (70/80) became “high responders” (&gt;1,000 AU/mL), and of these, 79% (63/80) mounted a “robust response” (&gt;4,160 AU/mL). Baseline antibody level, dialysis therapy, and hypoalbuminemia were independent predictors of impaired antibody response. <b><i>Conclusions:</i></b> A third dose of BNT162b2 COVID-19 vaccine, 6 months after the standard two-dose vaccination regimen, substantially improved humoral response in hemodialysis patients.