As a growing number of coronavirus vaccines advance through clinical trials, wealthy countries are fueling an extraordinary gap in access around the world, laying claim to more than half the doses that could come on the market by the end of next year. While many poor nations may be able to vaccinate at most 20% of their populations in 2021, some of the world’s richest countries have reserved enough doses to immunize their own multiple times over. With no guarantee that any particular vaccine would come through, these countries hedged their bets on a number of candidates. But if all the doses they have claimed are delivered, the European Union could inoculate its residents twice, Britain and the United States could do so four times over, and Canada six times over, according to a New York Times analysis of data on vaccine contracts collected by Duke University, UNICEF and Airfinity, a science analytics company.