I agree with Pfizer; a booster shot will be necessary. It would be beneficial if it existed now.
The reasoning is given in Delta Strain; the Rough Ride Begins, with links to napkin calculation #3.
While CDC/FDA are dedicated to science, their response to Pfizer is uncharacteristically brittle. Their difficult task is convincing refusniks that any vaccine now is better than none, or a later, improved one.
The dialog is abnormally simple, but we will get the booster.