Anti–receptor binding domain (RBD) antibody responses in plateletpheresis donors receiving COVID-19 messenger RNA (mRNA) vaccines. (A) Study schema. COVID-19 infection-naive CD4⁺-low (teal) and CD4⁺-high (orange) platelet donors were administered 2 mRNA vaccine doses per the schedule shown. (B) Anti-RBD IgG levels at baseline (V0) and after (V1, V2) each dose of vaccine. **P < .01; ****P < .0001.