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The girl is from central New Jersey. Her identity has not yet been released, and authorities say she died in December.
The health department says more than 4,000 people in New Jersey have contracted the flu this season, including 18 severe cases.
The flu outbreak is hitting hard across the country.
Flu is a contagious respiratory illness, spread by a virus. It can cause a miserable but relatively mild illness in many people, but a more severe illness in others.
Young children and the elderly are at greatest risk from flu and its complications. In a bad season, there are as many as 56,000 deaths connected to the flu.
In the U.S., annual flu shots are recommended for everyone age 6 months or older.
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Health officials say this year's shot correctly targets the strains that are making Americans sick, including one causing most of the illness, a Type A H3N2 flu virus. Some reports have said the vaccine is roughly 10 percent effective, but exactly how well it is working won't be known until next month.
The same virus was the dominant flu bug last winter, when the flu season wasn't so bad. It's not clear why this season - led by the same bug - is so much more intense, some experts said.
(The Associated Press contributed to this report.)
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