I have entertained the idea that this "spike protein" could be in other things that they want the human body to reject.

Scenario A: The US has reliable intelligence out of China that they have developed a deadly bioweapon that utilizes the spike protein (not necessarily a cornoavirus). China has since put researchers to death. Who is to say their rational is truthful? Perhaps they found the leakers. So the next move of all other countries was to develop a "spike protein blocker" as quickly as humanly possible and get everyone to take it in case China decides to unleash it upon the world... after ensuring their own population was sufficiently protected with their own secretly-deployed shots (mixed in with their own mandatory vaccines).

Scenario B: A life-extension mRNA therapy has been developed. It utilizes gene therapy with a viral vector, and in particular one with the spike protein. The vaccine is to ensure that if people in the general public gain access to this therapy, injecting it into themselves will result in their deaths due to a cytokine storm.