(A) Serological S protein concentration in pre-pandemic controls (n=14) vs COVID-19 patients (n=64). (B) Distribution of S protein concentration in patients according to the time from the onset of disease symptoms. (C) Distribution of S protein concentration according to patients’ age. (D) Distribution of S protein concentration according to patients’ gender. (E) Distribution of S protein concentration according to the global COVID-19 severity. This latter was categorised based on the patients’ respiratory symptoms as follows: mild: no oxygen required; moderate: oxygen required; severe: admission to intensive care or high-dependency unit (non-invasive ventilation). (F) Analysis of correlation between the S protein concentration and the Abbott anti-SARS-CoV-2 IgG index. The latter indicates a qualitative measurement of anti-nucleocapside (N) protein IgG antibodies. (G) Distribution of the Abbott anti-SARS-CoV-2 IgG index according to patients’ age. (H) Distribution of the Abbott anti-SARS-CoV-2 IgG index according to the time from the onset of disease symptoms. (I) Distribution of the Abbott anti-SARS-CoV-2 IgG index according to the global COVID-19 severity. Graphs in (A–E) and (G–I) report individual values and means ± SEM.