More than 9,400 children visited hospitals and emergency departments in 2017 for diseases that could have been prevented by a vaccine, leading to parents and insurers being charged $55.5 million, according to a new report by Children’s Hospital Colorado.

Rising health care costs and a severe flu season drove up the costs of treating children without vaccinations in the state, which was $35 million in 2015, said Dr. Jessica Cataldi, author of the report and a pediatrician at the hospital.

Most of the children treated for a vaccine-preventable illness in 2017 visited a hospital or emergency department for influenza. About 460 children were hospitalized and 8,656 kids were treated in emergency rooms for the flu, resulting in $42 million in charges, the report said.

Two of the children hospitalized — an infant and a toddler — died with influenza, according to the report.

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Each year, only about half of the children in Colorado get the flu shot, which is recommended for everyone 6 months and older, Cataldi said.

“When we look at our youngest kids we know that we still are not doing great,” she said.

Last year’s flu season, which ran from fall 2017 to spring 2018, was the worst the state has seen with 4,650 children and adults hospitalized for the respiratory illness. So far, this year’s flu season hasn’t been as harsh, partly because the strain being circulated — H1N1 — is less aggressive than the 2017-2018 virus, H3N2.

The report, released by Children’s Hospital this week, only looks at the charges that come from treating children for vaccine-preventable diseases at the hospital and emergency departments. It does not examine “actual costs,” which include doctor’s visits, medication, lost wages and decreased productivity.

Overall, about 29 percent of children between 19 and 35 months were under-immunized in the state in 2017, according to Children’s Hospital.

The report comes as Washington state is dealing with a measles outbreak, and last month a Denver resident was diagnosed with measles after traveling abroad. In Colorado, there were one to two cases of measles reported per year between 2013 and 2017,.