Background
The 2005-2006 influenza season in St. Charles County was very similar to the previous influenza season in 2004-2005. Both seasons experienced "late" starts with the first influenza case reported 12/1/06; both peaked in late-February/early-March; and both had fewer cases than experienced in later years (764 for 2005 and 712 for 2006).

The age distribution for the County reveals that the highest number of cases occurred in the 5-14 years category. This may be due to reporter bias, as most of the influenza reporters this season were pediatric offices. The county health department intends to increase its publicity in recruiting reporters for the 2006-07 season.

Please note that these incidence rates may be inflated due to using 2002 census data for the zip codes. The overall incidence rate for the county is 222 cases per 100,000 people. This statistic used the 2004 population estimate of 320,734 citizens.

The pie chart below depicts the number of flu cases per zip code in St. Charles County. Although difficult to read, the chart clearly indicates the most heavily populated zip codes in the county also had the highest number of influenza cases.

Subtypes
Subtype A was the most frequently seen influenza subtype with 543 cases (76.3%). Subtype B had 58 (8%) cases and unknown subtype had 109 (15%) of influenza cases. As noted on the graph below, Influenza B cases were identified later in the season and were diagnosed more frequently than A's in the later months. Analysis of subtype cases per zip code revealed that no relationship or trend existed between where a flu case lived and which subtype the case developed. The more heavily populated zip codes have the highest case incidence rates of both subtypes.

The following graph shows school absenteeism during the current influenza season. It is interesting to note that the peak in absenteeism closely follows the peak of confirmed influenza cases. As mentioned previously, this peak occurred in late February, early March.