The Oxford-AstraZeneca COVID-19 vaccine seems promising. Here's why it's not yet approved in the U.S.

Confidence in the University of Oxford and AstraZeneca COVID-19 vaccine seems to be growing. New analysis released this week suggests that not only does the vaccine protect against symptomatic COVID-19 infections, it could also significantly curb transmission, making it a potentially very effective tool for bringing the virus to heel.

The vaccine is affordable, easy to produce and store, and has been approved by health regulators in the United Kingdom, India, and Europe, as well as several other countries. But it has not yet been approved by the Food and Drug Administration in the United States, where it's still undergoing late-stage trials. That's led to some harsh criticism. The Mayo Clinic's Vincent Rajkumar called the delay a "huge blunder," while The Atlantic's Conor Friedersdorf described it as "the most profound impingement on my liberty I've faced." Writer Matthew Yglesias lamented a lack of pressure from lawmakers and the U.S. media for speedy approval.

For starters, AstraZeneca hasn't submitted any paperwork to the FDA, so, in short, it can't grant approval for what hasn't been requested, notes Axios' Sam Baker:

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But there are more complex reasons behind the holdup. In the U.K., Business Insider notes, health regulators "rely more heavily on a company's data to make conclusions," whereas in the U.S., an independent advisory committee takes a look at data sets from both the FDA and drug companies before the FDA can make its final decision.

In this case, the FDA was concerned following a series of missteps by AstraZeneca, including the company's failure to alert the agency about a decision to halt trials after a U.K. participant fell ill in September, and a strange discrepancy and incomplete testing of promising — but mistakenly-administered — half-strength doses. There's speculation submission and approval will come in April, but until the U.S. gets clearer data, the vaccine will remain in regulatory limbo.