CD4 and infection: The CD4+ T cell response to N measured by AIM was higher in individuals recovered from symptoms than in individuals recovered from asymptomatic symptoms (P = 0.020), whereas the CD4+ T cell responses to S and M were not different (P = 0.782 and 0.346 for S and M, respectively) between subjects recovered from symptomatic infection and recovered from asymptomatic infection (Fig. 3b).