I guess it comes down to the legal interpretation of "Best Reasonable Efforts".
Assuming this aspect of Belgium contract law sufficiently resembles its US counterpart, the interpretation of variants of "reasonable efforts" is a rabbit hole with few rivals. In the book written by a leading authority on US contract drafting[1], the topic commands its own chapter:
Chapter 8 Reasonable Efforts and Its Variants
[0] per sections 18.4 and 18.5 of the AstraZeneca/EU contract linked to here
[1] A Manual of Style for Contract Drafting by Ken Adams (https://www.americanbar.org/products/inv/book/297140045)
> 1.9. “Best Reasonable Efforts” means (a) in the case of AstraZeneca, the activities and degree of effort that a company of similar size with a similarly-sized infrastructure and similar resources as AstraZeneca would undertake or use in the development and manufacture of a Vaccine at the relevant stage of development or commercialization having regard to the urgent need for a Vaccine to end a global pandemic which is resulting in serious public health issues, restrictions on personal freedoms and economic impact, across the world but taking into account efficacy and safety; and (b) in the case of the Commission and the Participating Member States, the activities and degree of effort that governments would undertake or use in supporting their contractor in the development of the Vaccine having regard to the urgent need for a Vaccine to end a global pandemic which is resulting in serious public health issues, restrictions on personal freedoms and economic impact, across the world.
On this basis, AZ can argue that they have indeed made reasonable best efforts to manufacture the vaccine within the EU: I believe most vaccine companies are struggling to fulfil their targets at the moment because of things like a shortage of supplies, worker sickness because of coronavirus, etc. It would seem to be difficult for the Commission to point to a company who's doing it any better except perhaps US-based companies.
Furthermore the contract does not mark their UK manufacturing facilities as being exclusively for the EU - AZ can then argue that this contract is naturally subject to available resources in their UK manufacturing facilities and so, given this is a condition precedent, their obligations under this part of the contract have not become due just yet.
1.9. “Best Reasonable Efforts” means (a) in the case of AstraZeneca, the activities and degree of effort that a company of similar size with a similarly-sized infrastructure and similar resources as AstraZeneca would undertake or use in the development and manufacture of a Vaccine at the relevant stage of development or commercialization having regard to the urgent need for a Vaccine to end a global pandemic which is resulting in serious public health issues, restrictions on personal freedoms and economic impact, across the world but taking into account efficacy and safety; and (b) in the case of the Commission and the Participating Member States, the activities and degree of effort that governments would undertake or use in supporting their contractor in the development of the Vaccine having regard to the urgent need for a Vaccine to end a global pandemic which is resulting in serious public health issues, restrictions on personal freedoms and economic impact, across the world.