Four Colorado children have died of complications from the flu since mid-January, making this the worst flu season for children in five years.

Colorado health officials said Thursday that two of the children had not been vaccinated, while the other two had received one of two recommended flu vaccinations.

Two of the children died in the Denver area and two died on the Western Slope. At least two had other serious medical conditions before getting the flu.

The state Department of Public Health and Environment says it's too early to say whether the deaths signal a bad flu season or whether the increase is random.

In 2003, the flu killed 12 children in Colorado. Since then, no more than two children had died in any single season.