As large parts of the world prepare for a second wave of coronavirus cases governments have pinned their hopes on the rapid development of a vaccine to provide a route out of the pandemic.

Pharmaceutical companies have moved with unprecedented speed and tens of thousands of people in more than a dozen countries — from Brazil to Saudi Arabia — have volunteered for human trials.

If the vaccines work, as early tests suggest, the button will be hit for immediate mass production at a scale and speed never seen before. But recent setbacks for at least one leading manufacturer have shown that the path to a proven and widely available shot may not be straightforward.