Drugs are being tested that could reduce symptoms and save lives. But, given the way drugs are developed, it's unlikely that any single medicine will be anywhere as potent as a successful vaccine.

AstraZeneca, which is working with the University of Oxford, hasn't said what the illness is. It will try to determine whether the illness is related to the vaccine, or just a chance event.

This is the sixth vaccine candidate to join Operation Warp Speed's portfolio, and the largest vaccine deal to date.