CD40LG and COVID-19: Moreover, COVID-19 confirmed patients had higher SARS-CoV-2 IgM concentration than IgG before the 15th-day post symptom onset and vice versa after the 15th-day post symptom onset [28], and patients developed peak IgM of 94.1% approximately 20–22 days and all of them finally have positive IgG approximately 17–19 days after symptom onset [7].