Dr. Brannon Traxler, Interim Public Health Director with South Carolina's Department of Health and Environmental Control, confirms there is no difference between the two doses.

"They are exactly the same, in terms of the chemical makeup, but what they do and their function is a little bit different," Traxler said.
The difference is in the response your body has to each one.

Third dose:

The administration is calling it a booster, but right now it will be a third dose of either the Pfizer or Moderna vaccines. It is the same formula. Pfizer is investigating a potential booster, which would be a different formula that might address expected variants, but for now that is not what people will receive beginning Sept. 20 once the FDA and CDC approve it.