Clinical trials of one of the most advanced experimental Covid-19 vaccines, which is being developed by pharmaceutical company AstraZeneca and Oxford University, were "paused" Tuesday after a volunteer developed an unexplained illness.

With billions of people around the world still suffering from the fallout of the pandemic and the global death toll nearing 900,000, a worldwide race for a vaccine is underway, with nine companies already in late-stage Phase 3 trials.