Poehling KA, Szilagyi PG, Edwards K, Mitchel E, Barth R, Hughes H, Lafleur B, Schaffer SJ, Schwartz B, Griffin MR. Streptococcus pneumoniae-related illnesses in young children: secular trends and regional variation. Pediatr Infect Dis J. 2003 May; 22(5):413-8.