The global coronavirus death toll approached 200,000 on Saturday as the United Nations launched an international push for a vaccine to defeat the pandemic. Governments around the world are struggling to limit the economic devastation unleashed by the virus, which has infected nearly 2.8 million people and left half of humanity under some form of lockdown. The scale of the pandemic has forced medical research on the virus to move at unprecedented speed, but effective treatments are still far away and the United Nations chief said the effort will require cooperation on a global scale.