OF ALL THE outcomes that might accompany a global pandemic, a historic stockmarket boom seems among the least likely. And yet on June 30th the S&P 500, America’s main index, ended the second quarter up by 20% on its March 31st close, its strongest quarterly performance in more than two decades. The rally has puzzled investors. Since March 23rd, the market has soared by 39%, even as the country’s official covid-19 death toll has climbed to more than 127,000. It is now just 8% below its historic peak. Why?