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The evidence of a very bad flu season is everywhere. This week, Arkansas reported nine probable flu deaths. In Kentucky, two school districts have closed, and Philadelphia's health commissioner reported 104 new cases, a record for a single week in that city.
Colorado, one of the first states hit hard this year, reported 1,579 new cases last week, a slight decline from the week before. The state's total confirmed cases is nearing 10,000.
Yet federal health officials say it's too early to tell if the season will be worse than usual.
How can that be?
''Flu seasons are always notoriously difficult to predict,'' says epidemiologist Stephen Ostroff of the Centers for Disease Control and Prevention. ''We got off to an early start and in comparison to most flu years, the number of states reporting widespread flu activity is considerably higher than we've seen in other years, but whether we're seeing an early season that will end early . . . we can't say.''
Every year, flu claims 36,000 lives in the USA and requires hospitalization for another 114,000.
The CDC tracks flu in four ways:
* By monitoring testing in 120 labs around the world to make sure that new viruses or changes in circulating viruses are detected quickly.
* By doing surveillance through a network of sentinel clinics for influenza-like illness.
* By tracking the percentages of deaths from pneumonia and flu-like illness in 122 cities.
* By estimating levels of flu activity reported by state epidemiologists.
CDC director Julie Gerberding says the agency also is investigating whether this flu season is having a harder impact on children and the elderly than usual and the effectiveness of the current vaccine. This year's vaccine isn't a perfect match for the flu strain causing most of the cases, although experts believe the vaccine offers some protection.
Neither the CDC nor most state health departments take an actual head count of flu cases. Diseases that look like flu can be caused by all sorts of viruses and bacteria, and hospitals don't always test every patient to find out what's causing the illness. ''In the absence of widespread availability of laboratory testing for influenza,'' Gerberding says, ''it is just not possible to know whether someone actually has influenza'' or some other illness.
The CDC posts weekly reports on its Web site, www.cdc.gov, along with a map that shows in bright red the states where flu is widespread.
Currently, 24 states are in red, most of them in the West, ''so we probably haven't seen the full impact of the season on the eastern half of the U.S.,'' Ostroff says. ''We can say the map will probably get a bit redder.''
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