Johnson & Johnson asked U.S. regulators Thursday to clear the world's first single-dose COVID-19 vaccine, an easier-to-use option that could boost scarce supplies. 

J&J's vaccine was safe and offered strong protection against moderate to severe COVID-19, according to preliminary results from a massive international study. 

It didn't appear quite as strong as two-dose competitors made by Pfizer and Moderna -- a finding that may be more perception than reality, given differences in how each was tested.

But the Food and Drug Administration is asking its independent advisers to publicly debate all the data behind the single-dose shot -- just like its competitors were put under the microscope -- before it decides whether to green light a third vaccine option in the U.S.

Dr. Peter Marks, FDA's vaccine chief, has cautioned against making comparisons before the evidence is all in.

"With so much need to get this pandemic under control, I think we can't ignore any tool in the tool chest," he told the American Medical Association last week. "We will have to do our best to try to make sure that we find the populations that benefit the most from each of these vaccines and deploy them in a very thoughtful manner."

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