Early agreements to purchase Covid-19 vaccines are beginning to pay off for the likes of Britain, the U.S. and Canada, but much of the world is waiting. Tempers are flaring over scarce supplies of the shots, with the European Union set to tighten rules on the export of vaccines in a move that risks a global protectionist battle between countries. And although there are ambitious efforts to ensure that the shots reach low- and middle-income countries, it’s unclear when that will happen. Public health specialists warn

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