CD40LG and COVID-19: The seroconversion from N-protein specific IgA (not IgM) to N-protein specific IgG within 30 days after the onset of COVID-19 symptoms and the IgG levels, which persist above the cut-off 90–120 days after the onset of COVID-19 symptoms, are the best metrics for the assessment of the seroprevalence and estimating the exposure to SARS-CoV-2 in populations [52,53,54,55].