The U.S. Centers for Disease Control and Prevention is backing away from its decades-old estimate of the number of people who die annually from seasonal flu, instead saying deaths vary widely from year to year.

Instead of the estimated 36,000 annual flu deaths in the United States -- a figure often cited to encourage people to get flu shots -- the actual number in the past 30 years has ranged from a low of about 3,300 deaths to a high of nearly 49,000, the CDC said on Thursday.

Flu really is unpredictable. We don't know what the impact of flu will be at the beginning of a particular season, Dr. David Shay, a medical officer in the CDC's National Center for Immunization and Respiratory Diseases, told reporters in a conference call.

The estimates do not take into account the H1N1 swine flu pandemic of 2009, but they do suggest that some flu strains are more deadly than others.

The long-held 36,000 estimate was based on data from the 1990s when H3N2 viruses were prominent, the CDC said.

Its new analysis suggests that years when H3N2 flu strains are strongest, flu-related deaths are 2.7 times higher than in years when H1N1 or influenza B viruses were prominent.

Because of that variability, it is more accurate to use a range of deaths, the CDC said, but that likely will change over time with better diagnostic tests and better ways of tracking flu deaths.

Because we have this very wide range of deaths -- from 3,000 to 49,000 -- it's really meaningless to say what happens in an average flu season, said Shay, whose analysis appears in the CDC's weekly report on death and illness.

What is consistent is that flu strikes the elderly the hardest. About 90 percent of flu deaths in the 31 flu seasons between 1976 to 2007 occurred in people over age 65.

Shay said better tracking methods and improved tests, which have changed over the past decade, will likely make it impossible to compare estimates over time.

The CDC said the best way to prevent flu-related deaths is an annual flu vaccine, and recommends that virtually everyone over the age of six months gets one.

And it said quick treatment with antiviral drugs like Roche's Tamiflu, or oseltamivir, can cut the risk of severe illness and death among older people and others who are at greater risk of dying from the flu.

Medicine CDC