I was curious about this so went searching online. Was wondering if the vaccine cocktail in the 2nd shot for Moderna or Pfizer was different in any way to the 1st shot. Turns out, in both cases, they are the exact same chemical contents and the same dosage for both shots. The difference is in the function of the shots.
The first shot serves as the primer for your body, to kick start your body's initial immune system response to the virus. And the second serves as an additional immune booster after your body has had time to respond to the first shot and you wait the 21 to 28 days in between.
So taken together they are a one, two punch to teach your immune system how to respond to the virus, but both shots are exactly the same in composition and dosage.