CD40LG and infection: Using the pre-determined thresholds for seropositivity for each antibody isotype, out of the 357 samples collected during early infection (< 14 days post symptom onset), we were able to correctly identify an additional 19 (5%) cases among the IgG negative samples by adding IgM, 21 (6%) by adding IgA, and 33 (9%) by adding both IgM and IgA.