My understanding is that the reason it was required to have the same first and second dose was to make it easier tracing if there was a problem with a particula

My understanding is that the reason it was required to have the same first and second dose was to make it easier tracing if there was a problem with a particular vaccine and to get the efficacy numbers manufacturers were promising. From what I heard, in normal circumstances is even recommend to give a booster vaccine shot from a different manufacturer.