The vaccine developed by the University of Oxford stops 70% of people developing Covid symptoms, a large scale trial shows.
It is both a triumph and a disappointment after vaccines by Pfizer and Moderna showed 95% protection.
However, the Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two. So it will still play a significant role in tackling the pandemic, if it is approved by regulators.
The Oxford researchers have performed the normally decade-long process of developing a vaccine in around 10 months.