The GMT anti-RBD IgG and ps-NA before the fourth vaccine was significantly higher among recipients without than with a breakthrough infection and was 2895 BAU/mL (95% CI, 3111–11013) vs. 61 BAU/mL (95% CI, 2–475), p = 0.02; 662 (95% CI, 512–4096) vs. 121 (95% CI, 1–1024), p = 0.02, respectively. Here, XK is linked to infection.