I think this is one of those “Can’t use reason” with unreasonable people situations.
“Good parrot! Want a cracker?”
Likely so; the phrase I like is, “you can’t reason someone out of a position that they weren’t reasoned into.”
Sadly, I had a similar experience with a relative.
He also became ever more obsessed with these things “he knows” and incapable of having a discussion without bringing these things up. And he would get really angry when I would fail to agree with him.
I tried suggesting that we avoid those topics. He agreed with that approach, but just couldn’t help himself.
I don’t want to get into one our political fights, but Obama this or Biden that. I was supposed to just let him say his piece while biting my tongue. Eventually that became a relationship that wasn’t worth having.
This “died suddenly” thing is one of the more annoying claims on social media lately. Of course it’s always exaggerated even more than the original claim - like “1500 young world-class athletes.” When pressed for sources or even names (I think I would recognize a few world class athletes), the results usually include people who didn’t even died, those who died from other causes including accidents, and “young” athletes who are old and obese drug abusers.
If he fears death himself because vaccines + being an athlete brings sudden cardiac arrest he can easily avoid it by taking the vaccine and giving up sport. Stop being an athlete!
And what would happen is that since this year there were four deaths of young athletes, the CT theorist would say, “But last year there were three. See, that’s a 33% increase!”
Everyone knows facts have a liberal bias.
I once referenced The Economist, and the person with whom I was speaking called it “leftist communist propaganda”
While I acknowledge The Economist is fairly liberal (American meaning) when it comes to culture issues, it’s one giant love letter to capitalism, open markets, globalism, and low-regulation.
I told him it was clear he’d never read the economist.
Remember, that which is claimed without evidence can be dismissed without evidence. You don’t have to prove it is wrong, it is for them to prove it is true.
Until your friend can put forward reliable evidence there is simply nothing to discuss.
Also, my go to in that situation (knowing that I’m possibly speaking with a CT believer) is to ask what form of evidence would cause them to change their mind.
If they are unable or unwilling to even engage in that discussion then walk away from the subject, you aren’t going to make any progress on it.
He provided this “evidence”:
Here’s an article from 2021 showing a 5 fold increase in young athletes compared to previous years.
I haven’t had time to look in to it, but am guessing it’s nonsense.
It may or may not be nonsense but in any case I see no mention of the covid vaccine in that article so It is hard to see why he is pinning it on that specifically.
No idea what sort of reputation “Real Time News” has in Israel, but since the article is in Hebrew, I’ll take a wild guess that he hasn’t read it but is depending on secondary sources to interpret it. Note that the link is to a discussion on “America’s Frontline Doctors”, headed by Simone Gold (convicted for her role in the January 6th Capitol riot) and featuring such distinguished physicians as Stella Immanuel, the “demon sperm” lady.
Personally, I’d want to see peer-reviewed published data before trusting such sources, but that’s how I roll.
Link to article discussing one of those on-field athlete collapses, which references similar cardiac incidents occurring in 1977, 2012 and 2018, i.e. before the pandemic and use of Covid-19 vaccines.
Tell your friend that Shape-shifting Lizard people exist everywhere. Here is proof - if it’s on the internet it is true.
the lizard people control the world through politicians among other things, such as the iluminati and the financial system.
Bombard him with shape shifting lizard people theories and links every time he brings up his conspiracy theory. Tell him that THEY are responsible for these athlete deaths. Mock him if he does not believe you.
It won’t change his mind even a little bit, but it might be fun.
What’s really sad about all of this is that the Covid vaccine (and, specifically, the mRNA technology it utilized) is being applied to cancer research.
We may eventually come up with vaccines for some common cancers, and yet there’s an entire faction of our society who is inexplicably trying to derail that progress.
Not having read the prior replies … I’d first suggest that the underlying premise be validated: has there been an increase in the rate at which young athletes are dying in the last few years?
If no, then … stop 
We actually have a vaccine for one kind of cancer - cervical cancer is strongly tied to HPV, and we have a vaccine for that. But since HPV is an STD, a lot of religious parents don’t want their darlings to get it.
Many things happened about the time COVID vaccines became available. Personally, I blame the rise of cardiac arrests on the shortage of toilet paper.