Chandler Webb's direct cause of death was swelling of the brain, his mother said.
"We're angry because we believe it's the flu shot that [caused] it," she said.
She declined an autopsy, reasoning that a pending brain biopsy is sufficient to determine a cause of death, if possible.
The six neurologists who treated Chandler Webb at Intermountain Medical Center in Murray did a battery of tests looking for everything from Lyme disease to the West Nile virus to sexually transmitted diseases but couldn't find a cause for his malady or a cure, she said.
"I've never been so scared in all my life to see my son go through so much," she said.
An Intermountain spokesman said Chandler Webb's doctors are barred by law from talking about individual cases. Utah Department of Health spokesman Tom Hudachko said the state is aware of the case but can't verify the cause of death or whether it is investigating. The health department has no record of any Utahn dying from a reaction to the vaccine, he said.
"Like with other medical procedures, there can be side effects and adverse reactions," he said. "In the vast majority of those cases the side effects are not very severe soreness at the injection site, low fever, achiness. Occasionally, yes, there are more severe side effects from receiving the vaccine."
But those serious side effects are very rare. Some 130 million people nationwide, nearly half of Americans, get the flu vaccine every year, said Sankar Swaminathan, chief of the division of infectious diseases at the University of Utah School of Medicine.
Of those, about 1,000 people a year report some kind of reaction, and 140 people .0001 percent of the total immunized report serious side effects, he said.
And though it isn't foolproof, taking the vaccine does protect against illness and death.
"It varies from season to season, but thousands of people really have deaths attributed to the flu," Swaminathan said. Even if a bout with the flu isn't serious for one person, he or she might spread the virus. "You might go to the grocery store and infect someone with the flu who might die from it," he said.
Though scientists nationwide staunchly support vaccines and their important role in public health, they have become a topic of some controversy in recent years. Lori Webb said she was aware of such concerns before her son became ill, though she chose to get a flu vaccine two years ago.
One of her son's doctors said a reaction like her son's was possible after a flu vaccination, and he attempted unsuccessfully to treat it, she said.
Chandler Webb, living in Murray with friends while his mother lived in Hurricane, was healthy and went to the gym five times a week before he got sick.
He was admitted to Intermountain's emergency room in Murray on Oct. 23, nearly delirious after a week of vomiting, headaches, chills and shaking. Less than 24 hours later, he fell into a coma and never woke, despite doctors' attempts to diagnose and treat him, said Lori Webb.
When her son was taken off life support, she cut his hair for the last time telling him the story of "The Three Billy Goats Gruff" the way she did when he was a child to keep him still.
"I can't describe how hard it is to lose a child," she said.
Though she said she won't get another flu shot, Lori Webb didn't discourage others from getting vaccinated. She instead encourages people to read labels about possible side effects.
"We can't wrap our heads around this," she said. "... it's so bizarre."