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A huge U.S. study of another COVID-19 vaccine candidate got underway Monday as states continue to roll out scarce supplies of the first shots to a nation anxiously awaiting relief from the catastrophic outbreak. Public health experts say more options in addition to the two vaccines now being dispensed — one made by Pfizer and its German partner BioNTech, the other by Moderna — are critical to amassing enough shots for the country and the world.