Chicago ? Despite the advent of a vaccine four decades ago, flu-related deaths in the United States have risen dramatically since the 1970s, and influenza now claims more lives each year than AIDS, researchers say.

The rising death toll is attributed largely to the nation’s growing number of elderly people, who are especially vulnerable to the flu.

Only about 65 percent of older people get vaccinated, and the annual shots do not protect aging immune systems as well as they do younger ones.

The U.S. death toll surged fourfold from 16,263 in 1976-77 to 64,684 in 1998-99, the Centers for Disease Control and Prevention reported. Flu deaths now average about 36,000 a year, up from 20,000 in previous estimates, the CDC said.

Health and Human Services Secretary Tommy Thompson said the news “that influenza may be taking an even larger toll than we have realized” underscores the importance of flu shots, especially for older people.

The study appears in today’s Journal of the American Medical Assn.

While drug breakthroughs in the mid-1990s helped tame AIDS and reduce the U.S. death toll from 51,000 in 1995 to about 15,000 in 2001, the main weapon doctors have against flu — vaccines — have proven disappointingly ineffective in the most vulnerable population, people 65 and older.

The death toll pales in comparison to that of the worldwide flu epidemic of 1918, which killed more than 20 million people, including 500,000 Americans.

But the new numbers frustrate public health experts who had hoped the development of flu vaccine about 40 years ago would have had a greater effect. Annual flu shots have been recommended for people 65 and older since the 1960s and for those 50 and older since 2000.