BRUSSELS—In a stinging rebuke to pharma giant AstraZeneca on Wednesday, the European Union announced plans to negotiate a massive contract extension for Pfizer/BioNTech’s COVID-19 vaccine, insisting the 27-nation bloc had to go with companies that had shown their value in the pandemic. “We need to focus on technologies that have proven their worth,” said EU Commission President Ursula von der Leyen. She also announced that America’s Pfizer and Germany’s BioNTech would provide the EU with an extra 50 million doses in the second quarter of this year, making up for faltering deliveries of AstraZeneca. In contrast to the oft-criticized Anglo-Swedish company, von der Leyen said Pfizer/BioNTech “has proven to be a reliable partner. It has delivered on its commitments, and it is responsive to our needs. This is to the immediate benefit of EU citizens.” Exacerbating the problems for AstraZeneca, Denmark decided on Wednesday not to resume use of its …