There are five classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM, and the expression of IGHG1 and IGHG2, the subtype of IgG, tended to be higher in the HD-COVID-19 group, whereas IGHM was higher in the HDs with vaccine group (Supplementary Figure 4). Here, IGHG1 is linked to COVID-19.