XK and infection: The analyses of NA over time revealed a significant increase from T0 to T1 against all tested viral strains in both SV (p < 0.001), and SIV (EU and Delta: p < 0.05; Omicron: p < 0.01) (Figure 2A–C), indicating that repeated exposure to the antigen, through either infection or booster vaccination, augments immune responses to SARS-CoV-2.