Pfizer developed multiple candidates, not all of which were self-amplifying. The one that they have progressed the furthest in the clinic is not self-amplifying. The 2P modification does not just stabilize the spike antigen against transition to the postfusion state after contact with ACE2, it stabilizes it in prefusion form generally. The Spike without the 2P modification is unstable to multiple conditions besides ACE2 contact and is harder to express in cells. It's also worth noting that this modification is also in the spike antigen encoded by Pfizer's vaccine, and that it was not discovered by either of these companies but rather by Jason McLellan's lab in collaboration with... (read more)
My response from reddit:
Pfizer developed multiple candidates, not all of which were self-amplifying. The one that they have progressed the furthest in the clinic is not self-amplifying. The 2P modification does not just stabilize the spike antigen against transition to the postfusion state after contact with ACE2, it stabilizes it in prefusion form generally. The Spike without the 2P modification is unstable to multiple conditions besides ACE2 contact and is harder to express in cells. It's also worth noting that this modification is also in the spike antigen encoded by Pfizer's vaccine, and that it was not discovered by either of these companies but rather by Jason McLellan's lab in collaboration with... (read more)