Atlanta ? The last time there was a flu strain mutation similar to the one sickening thousands of Americans this year, nearly 65,000 died.

And that was only five years ago.

Flu experts say it’s clear this flu season will be much worse than in the past few years, but they are not ready to predict it will be one of the deadliest in modern times. Epidemiologists at the Centers for Disease Control and Prevention don’t know how long this year’s flu season will last nor how many people it might kill or hospitalize.

Already, it is worrisome because several children have died, and some parts of the country are facing flu shot shortages and swamped hospitals. It is one of the earliest flu seasons in a quarter-century, but some flu outbreaks can peak as early as December, rather than February, which is the norm.

“I think it’s clear this is going to be a more severe season than the past couple of seasons,” said Dr. Keiji Fukuda, the government’s leading flu expert.

Some experts predict this year’s death toll easily could surpass the annual average of 36,000 deaths.

What’s not clear is how it will stack up in the full context of previous outbreaks, Fukuda said.

In the winter of 1998-99, the country was in the second year of the virulent Sydney flu strain. Like this year’s Fujian strain, the Sydney strain was genetically slightly different from previous type A strains, making it harder for immune systems to fight off the virus.

Type A flu viruses of the same class as the Sydney or the Fujian strains tend to cause much more severe seasons than other kinds of influenza strains, said Dr. Tim Uyeki, a CDC epidemiologist.

By the time the 1998-99 flu season ended, 64,684 people had died — more than the number of people who died from AIDS at its peak in the mid-’90s, according to research by the CDC.

Hospitals, overflowing with people sick with the flu, forced other patients out to free up beds. Local officials had to use their disaster plans to handle the crisis.

The outbreak was severe even though that year’s flu vaccine matched the Sydney strain exactly. But the elderly — who are at high-risk for severe flu complications — have aging immune systems that flu shots do not protect as well as younger people.

“In those years, there were more deaths,” said Fukuda.

This year’s flu vaccine does not exactly match the new Fujian strain, although disease experts say it is close enough that it will provide some protection.

A major mutation of a flu strain — rather than a slight variation — usually occurs every 10 years and can cause a flu pandemic — a worldwide outbreak. These very new strains are particularly successful in attacking people’s immune systems.

The 1918-19 Spanish flu pandemic is considered to be the worst in modern history, killing about 21 million people and making up to 40 percent of the world population ill. A 1957 Asian flu pandemic killed 69,800 Americans and a 1968 Hong Kong flu pandemic killed 33,800 in the United States at a time when a normal flu season killed around 20,000, the CDC said.

Health officials note the world is overdue for another flu pandemic. There is no sign of that happening this year.

For years, health officials have urged people to get flu shots, but never have Americans used all the vaccine produced. This year may be different.

The three makers of the traditional flu shot — Aventis Pasteur, Chiron and Evans Vaccine — say they have shipped all 83.4 million doses of vaccine and have no more supplies.

Now health officials, those in Colorado in particular, are urging healthy people under age 49 to use the new, and more expensive, FluMist nasal spray, which is still in abundance. The spray cannot be used by older people, infants or at-risk people with chronic ailments.