DENVER -- Colorado has set a modern-day record for the number of flu cases confirmed in the past five days. More than 1,100 people were reported with the virus between last Thursday and noon Monday.

Officials said they haven't seen an explosion of flu like this in the five years the Colorado Department of Public Health and Environment has kept reliable statistics.

The more than 2,600 confirmed cases reported to the health department are just a fraction of those who actually have the flu. Most aren't counted because they don't see a doctor, officials added.

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Last year at this time, there were less than 15 flu cases in the entire state.

"It's crazy," said Denver pediatrician Dean Prina, who saw 40 children Monday. "It started earlier this year and seems more intense. It's among the worst flu seasons I've seen in my 23 years."

The stateside school absentee rate was 7 percent last week, worse than the highest rate of a year ago.

Also Monday, Kaiser Permanente of Colorado said more than 50 people may have inadvertently been given a polio or pneumonia vaccine instead of the flu shot.

Six people may have been given polio vaccine in Englewood and 50 may have gotten a pneumonia shot in Lakewood, spokeswoman Jacque Montgomery said.

Neither vaccine should cause health problems and everyone involved will be offered the flu shot again, she said.