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.
The C.D.C. is now starting a formal surveillance for encephalopathy, or brain damage, associated with flu.
Olivia, too, was unvaccinated. “That’s one of the biggest regrets we have,” said Ting Wang, Olivia’s mother.
Olivia’s illness did not ring any alarm bells at first. On Jan. 21, she was sent home from school with a fever of 101 degrees Fahrenheit, and she tested positive for influenza A. Neither the antiviral drug Tamiflu nor medications that suppress fever helped.
Three days later, when her temperature was up to 104 degrees, a doctor at urgent care didn’t see any cause for worry and sent her home. Her parents, both engineers at Google, said that if her fever had been taken more seriously then, the outcome might have been entirely different.
That night, Olivia’s fever spiked to 106.8, so high that it was a first for the emergency medical technician who accompanied them to the emergency room. She vomited and seemed extremely sleepy — odd for a child who was “energetic no matter what,” said Ms. Wang.
“If she feels uncomfortable, she’ll be crying, she’ll be yelling, she won’t be sleepy,” Ms. Wang said.
Her instincts were right. At the hospital in the morning, Olivia did not wake up. Brain scans revealed widespread swelling and damage to her brainstem, thalamus and other regions.
The neurologist told Ms. Wang and Hui Li, Olivia’s father, that she would probably remain in a vegetative state and that she would need a ventilator for the rest of her life. Two other children at the hospital with similar brain damage related to A.N.E. had both died.
“It was like a hell,” Ms. Wang recalled of that moment.
Influenza may be primarily a respiratory disease, but it also can affect the brain, said Dr. Andrew Silverman, a pediatric neurologist at Stanford Lucile Packard Children’s Hospital and a lead author on the study.
“If a child with the flu becomes confused, stops responding or has seizures, that’s an emergency,” he said.
A hallmark of A.N.E. is damage to the thalamus, one of the most interconnected structures in the brain. It plays a central role in sleep, attention, memory and consciousness, so damage to the region typically results in a coma or profound unresponsiveness.
Dr. Silverman and his colleagues contacted doctors at 76 academic hospitals, requesting to review cases from October 2023 to May 2025. Most of the deaths occurred within the first week of illness, they found, and in children with a median age of 2.
Of the survivors, nearly two-thirds had at least moderate neurological disabilities 90 days after the illness started. But many of the rest returned to school, an unexpected finding.
“They can regain independence,” Dr. Silverman said. “A small minority can make full, remarkable recoveries.”
Which children survive and which do not is still a mystery. Dr. Silverman started the study after treating two children, one of whom died and the other who fully recovered.
“The same virus, the same diagnosis, and two completely different outcomes,” he said. “Something like that really stays with you.”
Dr. Jaime Fergie, director of infectious diseases at Driscoll Children’s Hospital in Texas, has seen only three cases of A.N.E. The first case he saw, he recalled, was a “perfectly normal, healthy 12-year-old” who came to the hospital with fever, headache and seizures. The boy died within three days.
Dr. Fergie later treated a pair of siblings who came in with severe symptoms; the younger sibling, who was 17 months old, died, while the 5-year-old survived.
“The issue here, of course, is that most of these children are not vaccinated,” said Dr. Fergie, who was not involved in the new research.
After a week at the Stanford hospital, Olivia finally opened her eyes and was taken off the respirator, but she was still unresponsive. A bacterial infection triggered neurostorms — terrifying episodes where her body went stiff as a board and her heart rate shot up to 200 — and sent her back into intensive care.
Despite the lack of progress, Stanford Hospital discharged her to a rehab unit at a hospital in Oakland, which sent her home after two weeks. No other rehab center would accept Olivia, nor would insurance approve the costs.
Ms. Wang said she had derived enormous support from a parents’ group on the Chinese social media app WeChat. She had been frustrated as American doctors refused to try the therapies the parents in the chat group recommended, including hyperbaric oxygen therapy and transcranial magnetic stimulation. (Neither has been shown to help with A.N.E.)
In late March, heeding advice from their WeChat friends, Olivia’s parents decided to take her to China, their homeland. Mr. Li’s sister, a professor at Harvard, worked her connections and got Chinese doctors to escort Olivia and her parents on the flight.
Olivia spent one week at a hospital in Foshan, then 40 days at another hospital in Hangzhou, where she slowly learned to swallow some water. She received acupuncture and hyperbaric therapy, with her parents fretting over every choice.
After that, she spent another two months at the Shanghai Sunshine Rehabilitation Center, where the doctors used focused ultrasound to disable the malfunctioning area of her thalamus. In mid-July, the family returned home to California.
Olivia had improved in China, but not as much as her parents had hoped.
Now at home, she can swallow some yogurt and banana, but she still needs a feeding tube. She cannot talk, and the dystonia she developed means she also cannot walk. She mostly communicates through grunts and screams.
Her parents are still at home with her, because the insurance company denied their request for an in-home nurse.
The most heartbreaking thing, Ms. Wang said, is that Olivia does not recognize her parents.
“But we also feel lucky that she’s still with us, and we can still touch her and kiss her,” she said. “And there is still hope that she recovers with some level of skills and memories.”
Apoorva Mandavilli reports on science and global health for The Times, with a focus on infectious diseases and pandemics and the public health agencies that try to manage them.
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