The study’s lead author, BU PhD student Julia Gallini (GRS’26), says the findings are concerning and suggest “we need to increase efforts to support families, diagnose CTE during life, and treat CTE-related symptoms.” Photo via iStock/mphillips007

CTE

Nearly 75 Percent of Former NFL Players Had Brain Disease CTE, According to New Boston University–Led Study

“We have to learn from this data to make the game safer,” says former football star Matt Hasselbeck

The pro football season is in full swing, with NFL stars racking up amazing plays and crucial points—and a lot of brain-rattling collisions and tackles.

When injury forces a player onto the sidelines, fans might lament their team’s chances of a win, but a new Boston University-led study paints a worrying picture of the long-term health consequences of playing a sport filled with so many hits to the head. In a paper published in JAMA Network Open, researchers estimate that nearly 75 percent of all NFL players who died between 2008 and 2023 had signs of the neurodegenerative disease chronic traumatic encephalopathy (CTE).

Over the past 20 years, research at BU’s world-leading CTE Center has shown how the disease affects proteins in the brain and disrupts cell function, tying the condition to a range of symptoms, including memory loss, aggression, poor impulse control, and slow movement. The center’s researchers have also found that CTE can cause dementia. Although CTE can currently only be diagnosed after death, BU research has pointed to repetitive head impacts, like those in high-impact sports, as a cause.

“Providing a precise estimate for the prevalence of CTE among deceased NFL players helps define the scale of the problem and informs priorities for the potential current and future medical needs of this community,” says Julia Gallini (GRS’26), a PhD candidate in biostatistics at the BU College of Arts & Sciences and School of Public Health and the study’s lead author. “With CTE consistently found in less than 1 percent of donors to community brain banks, an estimated 75 percent prevalence among NFL players who died over a 16-year period is concerning, and suggests we need to increase efforts to support families, diagnose CTE during life, and treat CTE-related symptoms.”

Tracking the Rate of CTE Among Former NFL Players

In a separate recent study, BU researchers analyzed the deaths of more than 1,700 NFL players who had died between 2016 and 2021, including 338 who’d donated their brains for research or evaluation. (The BU CTE Center hosts the largest brain bank of its kind in the world, with more than 1,800 brains that have been donated to help advance its research on traumatic brain injury and CTE.) The researchers found CTE in almost all the donated brains they studied and concluded that the rate of the disease in the full group of ex-players would be at least 25 percent—and perhaps as high as 97.7 percent.

That wide range reflected uncertainty about how representative brain donors were of the overall NFL population. Brain donors are more likely to have symptoms believed to be related to CTE, a concept known as selection bias.

For the new paper, researchers analyzed this selection bias to estimate the prevalence of CTE in players they didn’t directly study. This estimate was made possible by understanding the differences between NFL players whose brains were donated and those whose brains were not. Among the 387 deceased NFL players whose brains were studied, 362 (94 percent) had some degree of CTE neuropathology. Researchers then examined available records of death certificates from the National Death Index for all 2,362 NFL players from this era, which included data on how many died of neurodegenerative disease and had dementia.

Researchers also analyzed other factors associated with brain donation, including cognitive symptoms other than dementia, age at death, and position and years played in the NFL. From this analysis, they estimated that former players with cognitive symptoms were about five times more likely to have their brains donated by their family than those without such symptoms. They used this estimated selection ratio to adjust the 94 percent prevalence of CTE observed among brain donors to an overall estimate of 74.5 percent.

Making Football Safer

The researchers caution that the new study’s estimate of prevalence only applies to former NFL players after death. It does not predict how many living former players have CTE or how many active players are impacted. But according to former Seattle Seahawks quarterback Matt Hasselbeck, the numbers make tough reading—even if they offer a path to curbing the disease.

“It’s hard to learn what the guys who built the game have suffered through,” says Hasselbeck, an 18-year NFL veteran and three-time All-Pro selection who serves as the ambassador for the BU-led Diagnose CTE study, an effort to detect the disease in life. “I’m looking at this like getting a scouting report. Now that we know what we’re up against, we have a better chance of winning. I can’t take back the hits I took, but I can sign up for research and do whatever I can to help the scientists so they can find a cure in my lifetime. We have to learn from this data to make the game safer, because we can’t accept these numbers for the next generation of players.”

According to the researchers, not every player with CTE pathology will go on to develop symptoms, particularly in cases of early-stage disease. However, previous studies have shown NFL players develop dementia at four times the rate of the general population. No other environmental or occupational exposure ever studied is known to have increased dementia risk at death by that much.

“We recognize these findings may cause concern for the community of living players and their families,” says Yorghos Tripodis, a BU School of Public Health professor of biostatistics and the latest study’s senior author. “We want them to know that we and others are standing by to provide support, and that we have hope for those living with CTE. Discoveries from the brain bank have already brought us significantly closer to a diagnosis during life, at which point our most important goal will be to develop effective treatments for CTE.”

This study was a collaboration with researchers at Mass General Brigham and the Concussion & CTE Foundation. The work was supported by the National Institute on Aging-funded BU Alzheimer’s Disease Research Center, the National Institute of Neurological Disorders and Stroke, and the Concussion & CTE Foundation.