Serotype-specific data.

Significant differences between study years within a serotype include: *p = 0.05 compared to 2004. **p = 0.02 compared to 2004. †p = 0.03 compared to 2006. ‡p = 0.03 compared to 2006. Significant differences in disease severity between study years overall include: §most severe study year is used as reference. ##least severe study year is used as reference. #Discrepancies in totals are due to the 7 cases that were not serotyped and the single case that did not receive a serological response profile.