An interesting video from November says that the trials of Moderna & Pfizer differed from the Astra Zenica/Oxford vaccine by one detail that surprised me. (6:30 mins in)

The A-Z vaccine trials checked for infection constantly, while the Pfizer/Moderna looked for people showing symptoms of COVID. Which explains the early result differing, which even Dr Fauci reckoned long-term statistics would show a migration to a similar efficacy.