Trump’s argument against vaccines works because people no longer trust the authorities — the governments, the scientists, the medical professionals, etc. — who tell them that they are safe. The biased mainstream media, the liberal elite, lying politicians, activist judges, crony capitalists, politically correct academics, the conspiring government, scientists bought off by big business, hypocritical religious leaders: all are radically corrupt, motivated by self-interest, and radically untrustworthy. In such a situation, people’s realm of trust can become more tribal in character, focusing upon people of their own class, background, friendship groups, family, locality, ethnicity, nationality, religion, etc. and deeply suspicious of and antagonistic towards people who do not belong to those groups. This collapse of trust hasn’t occurred because the general public has suddenly become expert in the science behind vaccinations and discovered the authorities’ claims concerning vaccines to be scientifically inaccurate. The trust that has been lost was never directed primarily at such scientific claims. Rather, it was a trust in the persons and agencies that presented us with them.
I think this is all quite right, but I think there’s another important element to the story: the creation, largely through radio and television, of a distinctive class. When a complex or otherwise disputed issue arises, the media look for informants, and those informants they call “experts.” The problem is that the term conflates several varieties of expertise and non-expertise. An “expert in infectious diseases” from the Centers for Disease Control is followed by an “expert in the paleo diet,” who is then succeeded by an “expert in political polling,” and then the hour is wrapped up by a visit from a “relationships expert.”
Some of these people don't know anything about anything. Others have deep learning in rigorously maintained fields of knowledge. But they have all been folded into the class of “expert.” And so when some of them are proved to be empty heads in empty suits, the reputation of the whole class is compromised. And that’s how you get a situation in which all experts are distrusted — in which their very designation as possessing expertise is just a big red flag.