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COMMUNICABLE DISEASE PREVENTION AND CONTROL
Influenza Season 2009-2010 - Final Report
Summer 2009:
2009 influenza A (H1N1) cases started appearing in the United States late April 2009 and St. Charles County saw its first case on May 5, 2009. Throughout the summer months St. Charles County had 98 reported cases of influenza A; 10 of those were subtyped as 2009 influenza A (H1N1) cases and the rest were diagnosed using rapid tests, but presumed to be 2009 influenza A (H1N1) cases based on national surveillance.
2009-2010 Influenza Season:
- To keep with national timelines the 2009-2010 influenza season in this report is data from week 35 in 2009 through week 20 in 2010.
- From week 35 to week 20 St. Charles County had 2103 cases of influenza reported; 2063 (98.1%) were type A, 19 (0.9%) were type B and 21 (1.0%) were unknown type.
- The vast majority of these cases (97%) were diagnosed between September and December 2009.
- Only 19 cases were subtyped among those reported in St Charles County and all were 2009 A (H1N1) cases (see graph below).
- Nationally, using summary of surveillance data from the CDC (http://www.cdc.gov/flu/weekly/), 99.8% of influenza A cases this past flu season were subtyped 2009 A (H1N1) cases.
Graph1: Influenza cases by type and report week in St. Charles County
#The number of confirmed Novel H1N1 is not a true representation of the prevalance of Novel H1N1 in our community. The Missouri State laboratory did not routinely test 2009influenza A Novel H1N1. Therefore true 2009 H1N1 cases will be included among the Influenza Type A counts.
Age:
More than half (58%) of the reported cases are in the 05-14 age group (school aged children) and 95% of cases are under the age of 50 years.
Graph 2 and 3: Influenza cases and rates by age group for weeks 35 in 2009 through 20 in 2010.
Year Comparisons:
Influenza cases jumped early in the fall of 2009 peaking during early October with a season total of 2103 cases reported. We average about 800 flu cases a year in St. Charles County, this past flu season had almost 3 times those cases and almost as many cases as we saw during the busy 2007-2008 flu season (2113 cases).
Graph 4: Four year comparison of total number of influenza cases by report week.
Hospital Emergency Room Data:
Missouri and USAI areas in Illinois have access to a syndromic surveillance system called ESSENCE which looks at chief complaint data (not diagnoses) from hospital emergency rooms. It gives a better situational awareness of what is happening in the community. For instance, graph 5 below compares Hospital ILI ER visits with actual reports of flu cases. As you can see they match well. However, back in May 2009 when the pandemic 2009 Flu A (H1N1) first hit we saw a different situation (graph 6). At the end of April we saw a big increase of ILI in the hospitals, but we did not see the same spike in flu reports. This was also the time that the the pandemic was being announced so the ILI from ESSENCE gave us a better understanding of the impact the annoucement of the pandemic had on the public.
Graph 5: ILI and Flu Cases for 2009-2010
Graph 6: ILI and Flu Cases for winter and spring 2010
Pneumonia and Influenza Mortality Surveillance:
Nationalwide, according to the CDC, influenza-associated pediatric deaths have more than doubled this past flu season compared to 2008-2009 flu season (276 cases in 2009-2010 compared to 134 reported cases in 2008-2009). Fortunately there were no pediatric influenza-associated deaths in St. Charles County!
Pneumonia-related deaths* tend to rise during influenza season so the graph below looks at the percentage of pneumonia-related deaths in St. Charles County. The percentage of pneumonia-related deaths in the 2009-2010 flu season is similar to 2008-2009 and slightly lower than the percentage in 2007-2008 (7.7% in 2009-2010, 7.6% in 2008-2009 and 8.4% in 2007-2008) (see graph 7).
Graph 7: Percentage of Pneumonia-related Deaths in St. Charles County.
*Pneumonia-related deaths are defined as deaths where pneumonia, aspiration pneumonia or bronchopneumonia as the primary casuse of death. These are not necessarily related to influenza infection.
2010 – 2011 Vaccine Composition:
Based on antigenic characterization during the 2009-2010 flu season, the WHO and FDA recommend the 2010-2011 flu vaccine will contain A/California/7/2009-like (2009 H1N1), A/Perth/16/2009-like (H3N2), and B/Brisbane/60/2008-like (B/Victoria lineage) viruses. Therefore, this vaccine will contain the 2009 Influnza A (H1N1) virus and not a seasonal influenza A (H1N1) component.
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