Flu Deaths 'Unpredictable'

CDC: Yearly Flu Death Toll Ranges from 3,000 to 50,000

Aug. 26, 2010 – U.S. deaths from flu range from about 3,000 to about 50,000 per year, the CDC now calculates.

The new numbers come from analysis of the last 31 flu seasons by CDC medical officer David Shay, MD, MPH and colleagues.

For years, the CDC has been estimating the annual flu toll at about 36,000 flu-related deaths per year. But that average came from the years 1990-1999.

The H3N2 type A flu bug dominated eight of those nine flu seasons -- and the Shay study finds that H3N2 seasons are nearly three times as deadly as H1N1 type A or type B seasons.

So how many people really die of flu in an average flu season?

"There are very few average seasons," Shay said at a news teleconference. "That is one reason why CDC is trying to move away from a single number and move to a range over a period of time to give a better prediction of what flu means to a community."

What this really means is that the CDC wants out of the flu prediction business. For example, there's no way to say whether the approaching 2010-2011 flu season will be mild or severe.

It could be like the 1986-1987 season, with only 3,349 deaths. It could be like the 2003-2004 season, with 48,614 deaths. Shay says there's no way to predict.

Even if the season starts out with a rash of H3N2 illness, he says, there's just no way to tell whether the milder flu bugs will come on strong later in the season.

And the type of flu bug isn't the only factor. Some flu seasons are shorter than others, so fewer people get sick and fewer die. During some flu seasons, a lot of elderly people get sick. Since about 90% of flu deaths are in the elderly, more people would die.

"Flu deaths are a moving target," Shay said. "Among vaccine-preventable diseases, there is not another one that has this kind of moving target."

Over the 31 seasons analyzed in the Shay study, the average annual number of deaths due to flu-related causes was 23,607. But Shay warns against using this as a benchmark.

"A simple average fails to give the impact of flu," Shay says.

Only one thing about flu is predictable. If you get the annual flu shot -- now recommended for everyone who does not have a medical reason to avoid it -- your odds of getting the flu are dramatically lower.

Moreover, if you're vaccinated, you're far less likely to infect someone who might come down with severe disease, possibly adding to the year's flu death toll.

The Shay study appears in the Aug. 27 issue of the CDC's MMWR: Morbidity and Mortality Weekly Report.

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