The vast majority of people don't have the background to understand the mechanism of action of vaccines. 100 years ago the effects of vaccines were easily obser

The vast majority of people don't have the background to understand the mechanism of action of vaccines. 100 years ago the effects of vaccines were easily observable, but now these diseases are rare enough such that a regular person can't rely on observation alone to make a decision to get vaccinated. They need to trust that the doctors know what they're doing.