The EU are attempting to argue that the "best efforts" bit refers to developing the vaccine and that, now the vaccine is available, AstraZeneca must deliver. That is clearly not what the contract says. The preamble is very clear that AZ must use best efforts to build the capacity to deliver 300M doses.
Clause 5.1 also commits AZ to use its best efforts to manufacture the EU's initial deliveries within the EU. Clause 5.4 requires AZ to use its best efforts to manufacture within the EU and UK, and sets out the process if AZ wants to manufacture at sites outside the EU and UK.
The EU appears to be arguing that "best efforts" only applied until the vaccine was been developed. Bluntly, on a quick read of this contract, that interpretation is unsustainable. If the EU attempts to take AZ to a neutral court over this I would expect AZ to win. I could, of course, be proved wrong but I struggle to see how a court could possibly interpret this contract to mean what the EU says it means.
By publishing this the EU appear to have removed all doubt. They are in the wrong. Of course, they desperately want to blame someone else for the slowness of their vaccine rollout, but it is their own fault. They prioritised their processes over speed, forgetting that delays cost lives. Now, sadly, those who live in the EU are suffering a slow vaccination programme as a result of the EU's failures.