Poehling KA, Edwards KM, Griffin MR, Szilagyi PG, Staat MA, Iwane MK, Snively BM, Suerken CK, Hall CB, Weinberg GA, Chaves SS, Zhu Y, McNeal MM, Bridges CB. The burden of influenza in young children, 2004-2009. Pediatrics. 2013 Feb; 131(2):207-16.