They say it's the first pediatric influenza death in the county this year.
County Department of Public Health officials say the otherwise healthy child died from the H3N2 strain of influenza A.
"It is always tragic to have to report a child dying, especially from flu," said Dr. Bob England, director of Maricopa County Department of Public Health. "But as I've been talking about with our recent measles cases, no vaccine is 100 percent effective, and what protects us is not just the vaccine that we receive but the vaccinations of everyone around us. The more people we get vaccinated from these deadly diseases, the less chance they have of spreading."
The name, gender and age of the child wasn't immediately disclosed Wednesday.
Experts say the H3N2 strain is associated with more serious illness in children and older adults and has mutated, making this year's vaccine less effective against the strain.
The federal Centers for Disease Control and Prevention recommends that everyone 6 months and older get the flu vaccine.
For more information about the flu and its symptoms or where to find a flu vaccine in Arizona, visit StopTheSpreadAZ.org.
Pediatric #flu death confirmed in Maricopa County: http://t.co/ezdJfnMRIQ
— Public Health (@Maricopahealth) February 4, 2015
Copyright 2015 The Associated Press modified.