This has nothing to do with intelligence, and everything to do with cults, beliefs, fears, Russian disinformation campaigns, political alignment, etc.

I've seen intelligent people fall for crazy conspiracy theories. Once all those antivaxers became pro-Russia, which basically has nothing to do with each other, it became clear were all this disinformation was coming from (talking mainly about Central Europe here, but I'm sure it applies to US too)

Once all those antivaxers became pro-Russia, which basically has nothing to do with each other

Not necessarily the case, as Russia has a longstanding record of propagating public-health misinformation. See https://www.bbc.com/news/world-europe-39419560 for instance.

This has been a part of their national agenda since the Communist revolution. "If we can't improve our own country, at least we can fuck up everybody else's."