Investors will have to wait until the end of 2010 to judge whether AstraZeneca is justified in paying $15.6bn for MedImmune of the US, a top executive at the Anglo-Swedish pharmaceuticals company said yesterday.

John Patterson, executive director, development, vigorously defended the price offered but said it would likely only be in two-and-a- half years that they could assess the results based on new product launches and the successful progress of medicines currently in development.