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For weeks, Americans looked on as other countries grappled with case reports of rare, sometimes fatal blood abnormalities among those who had received the AstraZeneca vaccine against COVID-19. That vaccine has not yet been authorized by the FDA, so restrictions on its use throughout Europe did not get that much attention in the United States. But Americans experienced a rude awakening this week when public-health officials called for a pause on the use of the Johnson & Johnson vaccine, after a few cases of the same, unusual blood-clotting syndrome turned up among the millions of people in the country who have received it.