[cfield]

And, more specifically, the interpretation of that phrase (as it's defined in the AstraZeneca/EU contract) under the laws of Belgium by the courts located in Brussels, Belgium.[0]

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

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[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)