XK and coinfection: Co-infections with other respiratory viruses (coronavirus (229, 43 and 63), parainfluenza (1, 2, 3 and 4), human metapneumovirus, parechovirus, rhinovirus, RSV A/B, adenovirus and enterovirus) or atypical bacteria (Mycobacterium pneumonia) were not identified in these samples, suggesting that no other viral source of NA activity was responsible for these high IC50 values.