We speculate that this could be due to (a) a less mature adaptive immune system in infants and young children, (b) a lower need for T cell and B cell responses because of other compensatory immune mechanisms (e.g., innate immune response in the respiratory tract), or (c) virus-specific CD4+ T cells and BMem preferentially extravasating from blood and homing to tissues at the primary site of infection. Here, CD4 is linked to infection.