First the MMR is FDA approved and NOT an experimental.
BIG DIFFERENCE.
Second you are possibly talking about a condition of employment.I have never had an employer ask me if I was current on the MMR.
Coercion can be threats. When you threaten to fine players unless they show proof. That’s coercion.
No it isn’t.
Get a student aid job. Almost every college in the country requires students to be MMRed up. And hospitals generally require employees to be vaxed up, MMR, flu, you name it.
It’s not coercion. It’s following the CBA the players voted for. Their CONTRACT says the NFL can make rules and violations of those rules results in fines. Their UNION agree that the covid vax could be on the list of required behavior. NOT COERCION.