In some cases (certainly not all) the answer to your question is: Because you actually study the clinical trials. Because you read all the epidemiological studi

In some cases (certainly not all) the answer to your question is: Because you actually study the clinical trials. Because you read all the epidemiological studies. Because you try to understand the pharmacokinetics of the vaccines. And after then you make a (very hard) decision weighing all the risks and unknowns on both ends.