Distribution of Ab titers after vaccinations. Distribution of Ab titers after the second (A) and third (B) doses of vaccine (Vax). Ab titers were significantly higher after the third dose of vaccine than after the second dose (mean ± DS, 1620 ± 1116 U/mL vs 539 ± 840 U/mL; P = .003). Percentages of patients with different Ab levels after the second (C) and third (D) vaccinations. Only 15% (5/33) of patients had Ab titers >1000 U/mL after the second dose (C) compared with 58% (14/24) after the third dose (D) (P = .0006). S, Spike.