Olivia Yanxi Li was entirely typical for a girl her age and yet extraordinary. She liked gymnastics, dressing up as Elsa from the movie “Frozen” and watching the animated television series Peppa Pig. She was fluent in Mandarin and English, switching effortlessly between the two.

On Dec. 18, Olivia Li turned 4. Barely five weeks later, she slipped into unconsciousness, a rare and unexpected consequence of a flu infection.

She had developed acute necrotizing encephalopathy, a neurological condition that can result from the flu or other infections, including Covid-.

The condition is extremely rare. In the largest study of A.N.E., published last month, scientists found only 41 cases in U.S. hospitals from the last two flu seasons. But it is devastatingly fast and often fatal, even though most affected children were healthy before the infection.

All but one of the children who died were unvaccinated.

The study is a reminder that the flu is not always a minor illness. In the last U.S. flu season, the condition was associated with 27,000 deaths, 266 of them of children. Since collection of the data began in 2004, the number of pediatric deaths was the highest reported in any influenza season outside of the swine flu pandemic in 2009.

The new study does not prove that vaccination prevents the necrotizing encephalopathy, but other studies support that finding. A preliminary analysis suggested that of the 266 flu-associated child deaths reported to the Centers for Disease Control and Prevention over the last two flu seasons, only 20 percent had been fully vaccinated against flu.