Questions & Answers: 2006-07 Influenza (Flu) Season
Characteristics of Flu Season from October 1- December 9
When and where did the 2006-07 flu season start?
North Carolina was the first state to report regional flu (influenza) activity this season during week 44 (October 29-November 4, 2006). Regional activity is defined as increased flu-like activity or flu outbreaks in at least two (but fewer than half) of the regions in a state with recent laboratory evidence of influenza in those regions. Define flu=influenza at the beginning before starting to use the term flu.
So far, how severe has the flu season been?
Influenza activity was low overall in the United States between October 1 and December 9, 2006, but activity is increasing primarily in the southeastern states. Flu viruses have been reported in 37 states. To date this season, regional or widespread flu activity has been reported in six states located in the southeastern United States, including: Alabama, Florida, Georgia, Mississippi, North Carolina, and South Carolina. Widespread flu activity is defined as increased flu-like activity or flu outbreaks in at least half of the regions in a state with recent laboratory evidence of influenza in the state. Please visit
http://www.cdc.gov/mmwr
for an explanation of definitions used to describe flu activity.
What determines the severity of a flu season?
The overall health impact (e.g., infections, hospitalizations and deaths) of a flu season varies from year to year. The severity of a flu season is judged according to five sets of numbers tracked by CDC each year. A season’s severity is determined by comparing these measures against previous seasons, and considering the following numbers:
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the number of states affected by flu and the degree to which they are affected;
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the percentage of tests from labs across the country that are positive for flu;
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the percentage of deaths reported to be caused by pneumonia and flu over the expected percentage;
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the number of flu-related deaths among children; and
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rates of influenza hospitalization in children.
Where has the most flu activity occurred in the United States so far this season?
States in the southeast area of the U.S. have reported higher influenza activity than other areas of the country, particularly: Alabama, Florida, Georgia, Mississippi, North Carolina and South Carolina. Flu outbreaks have been reported from Alabama, Florida and North Carolina. According to surveillance data, most of these outbreaks of flu have occurred among children.
What flu viruses are circulating this season?
As of December 9, 2006, influenza A viruses accounted for 77.9 percent of all confirmed flu cases. Influenza B viruses accounted for 22.1 percent.
What is the main flu virus in the United States during the 2006-07 season?
Influenza A (H1) viruses are the most common virus identified at this point in the 2006-07 flu season. As of December 9, 2006, influenza A viruses accounted for 77.9 percent of all confirmed flu cases and the majority (95%) of the influenza A viruses subtyped are H1.
Has the flu season peaked yet?
No. The flu season most often peaks in February in the United States, but may peak anywhere from December through May.
How many people have died from flu during the 2006-07 season so far?
Exact numbers of how many people died from flu so far this season cannot be determined because states are not required to report flu cases or deaths to CDC. However, one of the ways that CDC gauges pneumonia and flu deaths is through the 122 Cities Mortality Reporting System. This system collects information on a weekly basis on the number of death certificates that are filed in 122 cities and the number of those that list pneumonia or influenza (this is abbreviated as P & I) as a cause of death. This system represents approximately one-quarter of the U.S. population, and allows us to track the percentage of deaths caused by pneumonia or flu. This information is then compared each week to a baseline (expected level) and epidemic threshold (the point at which we would say we are seeing a statistically significant increase in the percent of P&I deaths) estimated from the past 5 years of data.
From October 1 to December 9, 2006, the percentage of P&I-associated deaths has ranged from 5.6 to 6.3 percent, which falls below epidemic levels.
Is there a good match between the strains selected for the vaccine and the strains that are circulating during the 2006-07 season?
It is too early to tell how well the vaccine strains will match strains that will circulate this season. To date, influenza A (H1) viruses have been most frequently reported, and the majority of influenza A (H1) viruses characterized are well matched by the vaccine.
Eight of the 10 influenza A (H1) viruses in circulation this year were characterized as A/New Caledonia/20/99-like. This strain of virus was included in the 2006-07 vaccine as the flu A (H1) component. The only flu A (H3) virus characterized thus far this season was characterized as A/Wisconsin/67/2005-like, which was included in the 2006-07 flu vaccine.
The influenza B component of the 2006-07 influenza vaccine is B/Ohio/01/2005, which belongs to the B/Victoria lineage of viruses. Six of the 17 characterized influenza B viruses belong to the B/Victoria lineage of viruses. Three of these six viruses are similar to B/Ohio/01/2005. Eleven of the 17 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.
CDC will continue to test how well the vaccine matches flu viruses in circulation as more flu strains become available for study.
Flu Deaths in Children
How many children have died from flu during the 2006-07 season?
No influenza-associated deaths among children have been reported through the National Notifiable Diseases Surveillance System for the 2006-07 flu season.
How does CDC monitor hospitalizations and deaths among children with the flu?
CDC monitors hospitalizations among children through two population-based surveillance networks: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). CDC monitors laboratory-confirmed influenza-associated deaths among children through the National Notifiable Diseases Surveillance System.
 
