The US signed their AZ deal months before the EU and UK. They threw billions of dollars at every company they could find, readily paying high rates while other countries tried to negotiate lower ones. So I'm really not sure where this idea of yours comes from.
$1.2 Billion to AZ in May: https://www.hhs.gov/about/news/2020/05/21/trump-administrati...
EU's deal was signed in August: https://www.fiercepharma.com/manufacturing/az-nets-396m-down...
$1.95 Billion to Pfizer in July: https://www.nytimes.com/2020/11/10/health/was-the-pfizer-vac...
EU's deal with Pfizer was in November: https://www.pfizer.com/news/press-release/press-release-deta...