CD4 and infection: Significantly weaker CD4+ T cell responses were observed 1 year after naturally occurring infection than after vaccination: especially, alpha-, beta-, gamma and delta mutant S protein peptide-induced IFN-γ responses, but also alpha-, beta- and delta mutant-induced IL-2 responses and beta- and delta-induced TNF-α responses were significantly lower in subjects who had been infected 1 year before.