For years, as researchers have established a link between repeated impacts to the head and the degenerative brain disease known as CTE, one major question has loomed: Just how common is it — especially among high-risk groups, like professional football players?
A new study from researchers at Mass General Brigham, published Tuesday in the prominent medical journal The BMJ, seeks to establish a more accurate baseline of CTE prevalence among retired NFL players who have died. Of football players who appeared in at least one NFL game in their careers and who died between 2016 and 2021, the study found that a minimum of around 25% had CTE upon their death.
"We've known that this disease process can be caused by, in part, repeated traumatic brain injury. But we didn't know how common it was," said Dr. Daniel Daneshvar, the study's lead author.
Chronic traumatic encephalopathy, or CTE, is a degenerative brain disease that can only be diagnosed with a brain dissection after death. Because of that, most research about the disease has relied on brains donated to researchers by families or by individuals themselves, many of whom had seen or experienced symptoms, such as memory loss or major mood swings, that led them to believe they might have the disease.
That dynamic has long muddied the picture about the actual prevalence of CTE — especially for combat veterans exposed to traumatic brain injuries, or current and former elite-level athletes in contact sports like American football and ice hockey, where blows to the body and head are a regular occurrence.
Daneshvar and his colleagues drew on the large collection of samples at the UNITE Brain Bank at Boston University. The brains of more than 330 NFL players who died between the years of 2008 and 2021 have been donated to the bank. Of those, 93% have been diagnosed with CTE.
To better understand what proportion of all NFL players that sample represents, the study's authors created a list of every player who had appeared in at least one NFL game since 1949. They referenced demographic data and obtained death certificates for all deceased players from the Centers for Disease Control and Prevention to create a denominator list of 1,712 players who died in the same time period. The researchers then narrowed the sample to those who had died between 2016 and 2021, when the proportion of players whose brains were donated for study was highest.
The prevalence estimate of 25% is at the "low end," Daneshvar said. The actual figure could be higher, the paper noted, pointing to a number of CTE diagnoses made outside of the brain bank sample, along with other suspected cases of CTE in former football players whose brains were not evaluated after death. Additionally, researchers wrote, an analysis of death certificates of football players who did not donate their brains revealed another 215 whose certificates listed a neurodegenerative disease as a cause of death.
At the extreme upper end, the study found that a maximum possible prevalence of CTE — if researchers assumed that every former football player had CTE except for the 20 whose brains had definitively been cleared of a diagnosis — was around 97%.
"The true rate of CTE at death in NFL players is somewhere in the middle," Daneshvar said.
In a statement to NPR, an NFL spokesperson said the league "continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts."
"The NFL remains committed to ensuring that the NFL community has access to a robust — and expanding — set of resources to enhance their physical and mental wellbeing. We encourage former players to utilize these resources to identify and seek treatment when they are concerned about their health," the statement continued.
A secondary aim of the study was to examine the link between dementia and CTE. Because the disease cannot be diagnosed in living patients, the relationship between symptoms in life and a diagnosis after death is still fuzzy.
The study found a high correlation between dementia in life and a diagnosis of the most advanced stage of CTE, Stage IV, after death; more than 90% of Stage IV samples were also diagnosed with dementia by a clinician based on medical records and information obtained from loved ones.
But the link was far less clear between dementia and lower stages of CTE, or even those whose brains had been found to be free of the disease's characteristic buildups of tau protein. Some 40% of CTE-free samples were diagnosed with dementia.
That means that further research is needed to best help the thousands of current and former NFL players who are still living, said Rachel Grashow, a neuroepidemiologist at the Harvard T. H. Chan School of Public Health who was not involved in the study.
"If we're trying to think about how to practically apply this information to living former NFL players who've just left the league, or those who left a while ago and are starting to age, that part deserves further discussion," said Grashow, who studies football player health. "The next step in centering the experiences of the living former players is to encourage them to speak with their doctors and loved ones about any concerning symptoms."
Conditions like sleep apnea, high blood pressure and low testosterone — all of which are common among former NFL players and tied to prior head injury — can lead to symptoms commonly associated with CTE, she said, with one important difference: The former conditions are treatable, while CTE is not (for now).
"Our concern is that the public and kitchen table conversations around CTE are creating hopelessness in living former NFL players. And this hopelessness may discourage them from reaching out to their doctors and getting treated for conditions, like sleep apnea, that are causing worrisome symptoms," she said.
Dr. Randall Chesnut, a neurological surgeon at the University of Washington who was not involved with the research, said the paper is "significant."
"It's really well done. It's a large population. It's well controlled within the limits of the statistical analysis that they can pull off on this," said Chesnut, an expert in brain- and spine-related sports injuries who also works as a concussion spotter in professional rugby matches.
Another caveat, Chesnut pointed out, was the average age of the cohort. Among the former players whose brains were donated, the mean age at death was around 66 years old, meaning most of their professional football careers took place before 2005, when CTE was first diagnosed in a former NFL player.
As scientific understanding of the relationship between repeated head trauma and neurodegenerative conditions has grown, the league has taken steps to improve player safety, such as banning dangerous plays, like traditional kickoffs and helmet-to-helmet tackles, and funding research of safer equipment.
"It was captured during a time when there was delayed recognition by the NFL that concussion was a big deal," Chesnut said. "So this may not represent the NFL in 2026."
Degenerative brain diseases can affect anyone, even those without a history of brain injury. Among Americans over the age of 55, the lifetime risk of eventually developing any form of dementia is 42%, one study published last year in the journal Nature Medicine found.
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