Why? There's no requirement that you get a credential from a particular group in society before being allowed to distrust them. If such a rule really existed it'd be a field day for scammers. For example nobody would be allowed to doubt any claim related to Web3 if they hadn't already spent years writing smart contracts. Or they wouldn't be allowed to decide Tesla FSD isn't safe enough, because they aren't themselves self driving car researchers.

It's really smart to distrust vaccine providers and public health authorities. Having blind faith in them is bad. You don't need a credential to see that, you just have to observe that they constantly make strong claims in favor of vaccines that they later walk back when it's too late, without any consequences whatsoever.

Why do I trust the vaccines?

Because medicine is generally useful and functional.

Because doctors studied at the same university that taught me much of my profession.

Because nations in economic dispute with each other, nations with territorial disputes, even nations at war with each other, still all agree that the vaccines are a good idea.