CAMBRIDGE – Covid-19 is confronting humanity with its most severe test since 1918, when an influenza pandemic killed more people than died in World War I. Yet the top leaders of the world’s two largest economies, China and the United States, have failed the first round. The initial reaction of both Chinese President Xi Jinping and U.S. President Donald Trump was denial. Crucial time for testing and containment was wasted, and opportunities for international cooperation were squandered. Instead, after costly national lockdowns, the two leaders engaged in propaganda battles with each other. China’s foreign ministry blamed the U.S. military for the emergence of the coronavirus in Wuhan, and Trump called it the “Chinese virus.” Yet Covid-19 does not care about the nationality of the humans it kills, and no global response will succeed without some degree of cooperation between the U.S. and China. Bilateral relations were already deteriorating rapidly when the virus hit.