That's what has been causing more tension than the actual nukes for a while now - effective countermeasures, or ICBM countermeasures. If one party has an effective countermeasure against ICBMs, it takes away MAD and the one holding the countermeasure gets to claim military superiority.

Of course, I don't believe there is such a system, or if there is it's very local; if it comes to nukes, they will come from anywhere (e.g. nuclear submarines) and can end up anywhere. If there was an effective anti-nuke system, we'd see launchers pop up everywhere, or if it's a space based one, see many launches of those to get enough coverage (like starlink).