The conspiracy of conspiracy theories
The concept of conspiracy theory is said to have been launched by the CIA after the assassination of John F Kennedy. There were lots of different ideas floating around about who or what had killed the young president. And remarkably many mistrusted the current theory of the lone gunman and Marine veteran Lee Harvey Oswald, who was himself shot shortly afterwards by nightclub owner Jack Ruby.
To be called a conspiracy theorist is a kind of belittlement and mockery. There is no point in arguing with such an individual, because what she says is narrow-minded and absurd, most of it distorted and unreliable. One can ignore these people.
In other words, it is a very effective tool for eliminating criticism and questioning. Often it works by pointing to scenarios of real conspiracies, such as associations where members believe the earth is flat, or individuals who refuse all vaccines, blood transfusions, etc.
Then when criticism of a specific vaccine arises, such as newly developed mRNA preparations, etc., they are compared to anti-vaxxers and flat earthers. The critics' ideas are quickly invalidated because they are associated with these somewhat narrow groups. A few months later, news of all sorts of side effects comes out, and long afterwards the critics are vindicated, but by then it's too late.