All but two Arizona counties have reported cases of the flu in what is turning out to be an early and busy season.

A total of 581 flu cases of the flu have been laboratory confirmed by the Arizona Department of Health Services this season, state data show. At this time last year only 10 cases had been confirmed statewide.

The U.S. overall is experiencing its earliest regular flu season in nearly a decade, and the primary strain that is circulating tends to make people sicker than other types. The most common subtype of flu this season so far is H3N2, federal data show. The subtype is generally associated with more severe flu seasons.

Every county in Arizona, with the exception of Gila and La Paz counties, has reported cases of the flu. Pima County has had 62 confirmed cases so far this season.

There have been no pediatric deaths in Arizona this season.

The cases reported represent a small portion of the true number. Many people do not visit the doctor when ill, and doctors are not expected to run tests on all patients exhibiting influenzalike symptoms.

Influenza often peaks in January, February or even later.

The federal Centers for Disease Control and Prevention recommends everyone 6 months of age and older get a flu shot, except those who cannot be vaccinated for medical reasons.

Last season about 48 percent of Americans got vaccinated, officials said.

So far the strains of flu that are turning up appear to be an excellent match with the vaccine, federal officials have said.

Cases of flu have been reported in 29 states. As of Dec. 15, higher-than-normal reports of flulike illnesses had been reported in 12 states, most of them in the South and Southeast. They are: Virginia, Alabama, Georgia, Louisiana, Mississippi, Missouri, North and South Carolina, Tennessee, Texas, Illinois and Utah.

Based on data from the past two decades, during the years when H3N2 is the predominant flu strain, "there are more deaths and hospitalizations," said Joe Bresee, chief of epidemiology at the CDC's influenza division.

It is not completely clear why. One factor may be that the elderly, who are at high risk for flu complications, tend to become sicker with the H3N2 strain than the other two common flu strains, he said.

But this flu season offers two bits of good news, as well.

Of the flu strains that are spreading this year, about 80 percent are the influenza A type, and almost all of those are the H3N2 strain, Bresee said. That matches well with this year's flu vaccine, which includes the H3N2 strain.

Also, the Food and Drug Administration on Dec. 21 expanded the approved use of Tamiflu to treat children as young as 2 weeks old who have shown symptoms of flu. The drug was previously approved to treat adults and children 1 year and older. The dose for children under 1 year old must be based on their exact weight.

Flu is extremely unpredictable. "You never know when the peak will occur or how big the peak will be," Bresee said. "If you've seen one flu year, it's just one flu year."

About 112 million Americans had been vaccinated by the end of November, the CDC said.

On average, about 25,000 Americans die each flu season, according to the CDC.

"You never know when the peak will occur or how big the peak will be. If you've seen one flu year, it's just one flu year."

Joe Bresee, chief of epidemiology at the CDC's influenza division

Arizona Daily Star reporter Stephanie Innes and The Washington Post contributed to this report.