In intranasal challenge studies of seronegative healthy volunteers, pre-existing CD4+, but not CD8+, T cells responding to conserved IAV nucleoprotein or matrix protein epitopes were detectable at 7 days after infection when no serum Abs were detectable against the challenging virus and were significantly associated with reduced viral shedding, symptom scores, and illness duration [8]. This evidence concerns the gene CD4 and infection.