The influenza strain responsible for the 2009 worldwide pandemic appears to be making a comeback this year in Colorado, health officials say.

More Coloradans have been hospitalized for the flu so far this year than were hospitalized during the same period last year, and a majority of this year's cases are attributable to the H1N1 flu strain that emerged in 2009, said Lisa Miller, a Colorado Department of Public Health and Environment epidemiologist.

The majority of hospitalizations have been among the demographic that typically fares well during flu season.

Through Saturday, people ages 25 through 64 accounted for 56 percent of the hospitalizations across Colorado, according to the state health department.

Last year, 30 percent of hospitalizations were among that demographic.

"I wish I could explain flu every year and be able to predict it," Miller said. "Every year, it's a bit unique."

In El Paso County, 135 people had been hospitalized between Sept. 29 and Thursday, said Susan Wheelan, El Paso County Public Health spokeswoman. The county totalled 161 cases during the entire previous flu season.

Only hospitalizations and child deaths are tracked by health officials across the state, though some flu-related deaths have been reported by individual county health departments. For example, two middle-aged people recently died in Pueblo County, the Pueblo Chieftain reported.

Comparing flu statistics, though, is difficult because state and county flu numbers don't quite match up.

Through Saturday, state health officials counted 748 people hospitalized with the flu across Colorado, 97 of which were from El Paso County. The state tallied 506 hospitalizations at this point last year.

But El Paso County officials tallied 120 hospitalizations through Saturday, said Bill Letson, the county's medical director. The discrepancy could not be explained Thursday evening.

The uptick in flu cases does not prove that the new flu shot introduced this year isn't working, Miller said.

The new vaccine protects against four strains of the flu, rather than the traditional three. But the new strain added to many of this year's flu shots was meant to protect against a "B" strain of the virus.

The H1N1 strain - which has proven most prevalent this year - is an "A" strain, Miller said.

Still, the best means of defense against the flu is to get vaccinated, Letson said, especially because the vaccine worked "reasonably well" when the H1N1 strain circulated in 2009.

"First and foremost, people should get the vaccine, just do it," Letson said. "And because it (H1N1) tends to be a bit more severe virus, it just makes all the sense in the world for people to be vaccinated."