When we further analyzed the specific CD27+ memory B cell subsets, we found that Wuhan Hu-1 RBD-specific CD27+ memory B cells and, in particular IgG+CD27+ and IgA+CD27+, but not IgM+CD27+, were significantly higher at 9 months in previously infected subjects compared to infection-naïve subjects (Figure 2C). Here, CD27 is linked to infection.