GMT: Geometric mean titer; P: Positive; N: Negative; Groups: I. Positive for SARS-CoV-2- and IBV-specific IgG; II. Positive for SARS-CoV-2-specific IgG; III. Negative for SARS-CoV-2- and IBV-specific IgG; IV. Negative for SARS-CoV-2-specific IgG and positive for IBV-specific IgG