In practice it's likely that we'll do both. That is, all the doses will be used, and none reserved, as they arrive according to a priority schedule (ICU staff,

In practice it's likely that we'll do both. That is, all the doses will be used, and none reserved, as they arrive according to a priority schedule (ICU staff, care home staff, care home residents first, etc). Then when it's time for the second dose, the high priority targets will get their second dose immediately as new doses arrive while lower priority targets may have their second dose delayed as a first dose is given to those even further down the priority list. The result being that those at highest risk get the double dose ASAP while the twenty year olds get their first dose at the expense of the forty year olds having to wait a little longer for their second dose. This is because even a first dose while not fully protective is protective against serious covid-19 outcomes which is a big deal in itself. According to this scheme twenty year olds will be getting their second dose sometime in their thirties. :)