It's considered unethical (and actually unlawful) to purposely expose a healthy individual to an illness/ pathogen etc. so they simply give what ever nonsense vaccine and then 'study' if someone gets target illness. In the cases of people already exposed - they 'study' them recovering after giving the vaccine and insist any benefit is vaxx related. It's all bait and switch and depending on study parameters and how the data is collated and presented - anything can become anything.