AMNA NAWAZ: At least one in four NFL players has CTE when they die, that is the concerning conclusion of a new study out today that examined the brains of former pro-football players.

Chronic traumatic encephalopathy, known as CTE, is a brain disease thought to be caused by years of sustained head trauma.

Symptoms can include depression, anxiety, and aggression, and, later in life, memory trouble and dementia.

Because CTE can only be diagnosed after an autopsy of the brain, the study's researchers say the actual number of NFL players suffering from the disease may be even higher.

For more, we're joined by Dr.

Chris Nowinski.

He's a former college football player, co-founder and CEO of the Concussion and CTE Foundation, and a co-author of this study.

Dr.

Nowinski, welcome to the show.

Thanks for joining us.

CHRIS NOWINSKI, Co-Founder and CEO, Concussion & CTE Foundation: Thanks for having me.

AMNA NAWAZ: So, in the study, you focused on about 235 donated brains from former NFL players who died between 2016 and 2021.

I mean, did you expect the rate of CTE to be so high?

Tell us more about what you found.

CHRIS NOWINSKI: Yes, I mean, ever since we started studying NFL players' brains, it's been more than 90 percent of the sample.

But, over time we got much better at getting a larger sample of NFL players and built a partnership with the retired community so much that we got more than 25 percent of NFL players' brains.

Unfortunately, that rate is still more than 90 percent in that sample.

We have always hoped that this disease might be rare, but I think this is that final nail in the coffin to say, no, this is absolutely not a rare disease in the NFL population.

And now we know it's somewhere between 25 percent and 98 percent.

And we need to figure out what it is between those two numbers.

AMNA NAWAZ: So that one in four number we're seeing is likely a conservative estimate?

Is that what you're saying?

CHRIS NOWINSKI: It's it's almost certainly an extremely conservative estimate.

AMNA NAWAZ: Do you know... CHRIS NOWINSKI: Unfortunately.

AMNA NAWAZ: Do you know from these relationships you have with the families and former players if any of those players or all the players who had CTE also had symptoms they exhibited before their deaths?

CHRIS NOWINSKI: Yes, unfortunately, nearly two-thirds of them had dementia while they were alive.

So most of these were very severe, most of them were very impacted, and that's why their families reached out.

But we are also able to go back and get the national death records of every NFL player we didn't get.

And the unfortunate thing is, we got far fewer than half of NFL players who died with of a neurodegenerative disease.

AMNA NAWAZ: Obviously, this is resonating across a lot of people.

But we did speak to a former NFL player today.

That's Randy Grimes.

He played in the league for nearly a decade.

And we asked him about his reaction to the study earlier today.

Here's what he had to say.

RANDY GRIMES, Former NFL Player: They're tested in the brains of former NFL players, and a lot of them had a lot of years in the league.

But we're not testing the brains of high schoolers, people that played a couple of years in college.

So I think there's a lot more research that needs to be done.

But at least we're talking about it and we're moving in that direction.

AMNA NAWAZ: Dr.

Nowinski, there are millions of people who played as young players in youth leagues, who played in high school, who played in college.

What should we take away from your study about them?

CHRIS NOWINSKI: Well, yes, I know Randy.

He is a great guy, and he's helped us with supporting families.

I guess he -- I do need to let him know, though, we are also studying college football players.

We have studied nearly 400 of them, and we have studied more than 100 former high school players.

What we have shown through the study of people with different career lengths is a clear dose response relationship.

We're -- over 90 percent are NFL players, but I think we're still around two-thirds of our college players we have studied have had it, and about a third of our high school.

We know, again, that sampling bias, so the numbers got to be lower than that, but it's still far too many, considering 20 million men have played high school football in this country.

And so I hope this is the moment where we shift the conversation to this is a minority of players, and we don't have to worry about this, to one where we say, OK, this is a public health crisis that we need to confront, not only in the NFL, but throughout football.

And there are a lot of men who are going to need treatment and a cure.

And we need to really ramp up our investments in making this a curable disease in our lifetime.

AMNA NAWAZ: I just want to put to you part of what the NFL sent to us in a statement in response, because they stress that they are continually striving to make football safer.

They talk about implementing strategies to reduce concussions and head impacts.

That includes new rules, new helmet technology.

They also point out the average age of the players that you studied when they died was about 66, noting they hadn't played for many years.

So is it possible your findings reflect the impacts of a different, earlier era of football?

Is the game safer today?

CHRIS NOWINSKI: Is it possible?

Yes.

Is it probable?

No.

We have made some modest changes to the game, and that's great.

But also players are bigger, stronger, and faster than they have ever been, and they're starting younger, and seasons are longer.

So I wouldn't be surprised if it's worse today or it peaked very recently.

So we have a lot of work ahead of us, and we need to go a lot further if we want to tell ourselves that this game is safe enough, because, as a former player, I will tell you I'm still uncomfortable with this.

And just to give you one example, one of our latest cases was Marshawn Kneeland, who played for the cowboys just over a year ago.

At 24 years old, he died by suicide.

He already had CTE, and he played in the modern era of concussion protocols and practice restrictions.

So nobody should be resting on their laurels thinking that we have this covered and this is now something in the rearview mirror.

AMNA NAWAZ: I mean, you did play at a high level yourself.

The question a lot of folks are going to be asking, in your opinion, is the risk worth it to play today?

CHRIS NOWINSKI: That's something I struggle with.

I will tell you, I just -- I have keep these on my desk.

These are the -- this is me on the cover of the 99 Harvard football program.

My teammate Chris Eitzmann was the last person in the study.

He was our captain.

He died December 29, 2021.

He had stage two disease.

This is our strong safety, Michael Brooks.

He died a couple of years ago.

He had stage two disease.

Both of their deaths seem to be related to their brain disease.

So I -- from my perspective, all I deal with is struggling football players.

So I'm probably not the right person to ask, but this issue is far worse than the conversation we're having at our kitchen table.

AMNA NAWAZ: Dr.

Chris Nowinski, thank you so much for joining us tonight.

We appreciate your time.

CHRIS NOWINSKI: Thanks for having me.