(CNN) -- Arizona reported its fourth death from the H1N1 virus, commonly known as swine flu, health officials said Wednesday.
The H1N1 strain is relatively mild, but it's being closely watched for mutations that might become severe.
The victim was the second child in the Pima County area to die of flu complications, health officials said. The age of child who died Wednesday was not released, but the patient's health was "medically compromised."
Earlier, officials in Cook County, Illinois, recorded their second death from H1N1. The latest victim, from suburban Chicago, died within the past couple of days and had "significant underlying medical conditions," said spokeswoman Kelly Jakubek.
The first fatality occurred in a Chicago resident over the weekend. That victim also had underlying medical conditions, Jakubek said.
Citing family privacy, she would not reveal the victims' ages or genders.
By late Wednesday, the most-recent deaths in Arizona and Illinois had not been included in the Centers for Disease Control and Prevention's official tally of 11 U.S. deaths from the disease. They are in Arizona (three), Missouri (one), New York (two), Texas (three), Utah (one) and Washington (one).
Nor did the CDC's count include two more deaths reported by the New York City Department of Health and Hygiene.
The World Health Organization has counted 14,557 cases of H1N1 -- 96 of them fatal -- in 48 countries.
Seasonal flu typically kills 36,000 Americans in any given year, though such cases usually have tapered off by this time of year, according to the CDC.
Though the H1N1 strain is considered relatively mild, public health officials have been scrutinizing its spread since it was first identified in April in Mexico because they are concerned it could mutate and become more severe.
All About Swine Flu • Centers for Disease Control and Prevention • Arizona • Illinois