https://www.cnn.com/2021/03/17/politics/us-astrazeneca-mexico-canada/index.html
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There are tens of millions of AstraZeneca doses stockpiled in the US and the company believes it will have roughly 50 million Covid-19 vaccine doses available to the US government by the end of April. None of those doses are available to Americans now because AstraZeneca has not applied to the Food and Drug Administration for an emergency use authorization, and the vaccine is still going through clinical trials in the US.
AstraZeneca has been approved for use in both Canada and Mexico, and the company itself has asked the Biden administration to consider requests to donate inventory to other nations. "
I have no idea why they haven't applied for an emergency use authorization.