IL2 and infection: Interestingly, while the number of RBD-specific IgG producing B cells in the heterologous vaccination group was similar to that in individuals with three doses of mRNA vaccine, or with mRNA vaccine booster after natural infection, the S1-specific T cell response (represented by the number of IL-2 or IL-2/IFN-γ producing T cells) induced by the heterologous vaccination seems to be the strongest among all vaccination/immunization strategies tested (Fig. 3a–d).