Figure 1 (A, B) Decrease in delta (δ) neutralizing antibody (NA) titers after baseline immunization, followed by their increase after mRNA booster. The geometric mean titers (GMT) and the prevalence of δ NA titers >1:10 are tabulated. (C) Renewed decline in titers. Four weeks after the third mRNA immunization, δ NAs tended to be higher than after the second immunization (p = 0.27).(D, E) Increase in CD4+ and CD4+/CD8+ T-cell reactivity after mRNA booster. (F) Renewed decrease in T-cell reactivity. ****: p < 0.0001; ***: p < 0.001; * p < 0.05. Horizontal dashed lines indicate test cut-off values.