The idea behind the second dose being delayed in the UK was it was better to give 24 million and 80% coverage to the most vulnerable people with 1 dose over 12 weeks (assuming 2 million per week), then to give 2 doses to 9 million and 1 dose to 3 million over 12 weeks
.8 * 24 = 19.2m covered in scenario 1
.959 + .83 = 10.95m covered in scenario 2
Even if it were 60% with 1 dose and 95% with 2 it would be
I imagine "fairly rapidly" means a few months rather than a few weeks, in which case delaying the second dose from 4 weeks to 8 weeks won't cause any dramatic dips (this is all conjecture though).
