CD4 and infection: CD4+ T cell responses to N peptides were detected also in some participants before the infection (N=11), but the activation levels were significantly lower compared to the levels seen after a breakthrough infection (p<0.0001 at 3D3mo, p=0.0005 at 3D8-12mo, p=0.0001 at 4D3wk, and p=0.0498 at 4D3mo).