The toll has climbed from about 20,000 deaths a year in the 1970s to an average of about 36,000 a year in the '90s -- with the number soaring as high as 70,000 in bad years, according to the study in today's Journal of the American Medical Assn.
Although more virulent strains of the influenza virus have played a role in the increase, the most important factor is the aging of the U.S. population. The number of Americans over 80 has doubled during the last three decades, said Dr. Keiji Fukuda of the CDC.
About 90% of all flu deaths occur among people over 65, and those over 80 are 32 times as likely to die from side effects of influenza as those from 60 to 65, he said.
As the population continues to age, "Simple demographics practically ensure an impending public health disaster of great proportion," Dr. David H. Morens of the National Institute of Allergy and Infectious Diseases wrote in an editorial in the same issue of the journal.
Nonetheless, a third of older Americans do not receive annual flu shots, even though the shots are free under Medicare, said Tommy G. Thompson, secretary of the Department of Health and Human Services. Many of those who refuse the shots suffer from the pernicious -- and unfounded -- belief that the shots actually cause the flu, CDC studies have shown.
Vaccination rates are much lower -- around 30% -- among younger people who are at high risk of dying from influenza because they already have such diseases as diabetes, asthma, lung conditions, heart disease and HIV infections, Fukuda said.
During the last two years, manufacturing difficulties have limited the supply of influenza vaccines from 70 million to 75 million doses a year, creating shortages in some areas. This year, Fukuda said, about 94 million doses are available, which should be enough for nearly everybody who wants one. The CDC recommends vaccinations for everyone over the age of 50, those with medical conditions that put them at high risk, family members of those with high-risk conditions and health-care workers.
The CDC recommends vaccinations be given in October and November, before the flu season begins, but "it is definitely not too late to get your flu shot if you have not received one," Fukuda said. This year's flu season has not yet peaked, he said.
The new study also looked at deaths caused by respiratory syncytial virus, which produces symptoms that are often mistaken for flu. RSV infections are the most common cause of death in infants under the age of 1, but the CDC team concluded that they also are important in adult mortality.
They concluded that RSV causes about 11,000 deaths a year, and that 78% of those deaths occur among the elderly. Unfortunately, researchers have not yet been able to produce an effective vaccine against RSV.