> and was wondering why we expected the immunity after the vaccine to last longer than natural immunity.

In the case of Pfizer and Astra Zeneca, the simple answer is that it is the second dose which presents a second challenge to the immune system which is designed to stimulate the longer protection, I'm quite surprised that none of your medical friends said that.

That leaves the question of how the single-dose jabs fair, and the honest answer is, I don't know.