No mRNA vaccine has ever been approved by regulators, but the Oxford approach has been used in vaccines given safely to thousands of people of all ages for diseases ranging from TB and malaria to Mers (another coronavirus) and Ebola, for which it is now being used in Uganda and the Democratic Republic of the Congo. Oxford opted for the chimp virus because it has the potential to generate a strong immune response and, since it cannot grow in humans, it should be safe.

Two other frontrunner vaccines, from BioNTech (developed with Pfizer) and Moderna (developed with the US National Institute of Allergy and Infectious Diseases), perform well, according to interim results from phase 3 human trials, achieving more than 90% efficacy. These are based on technology that is new to the vaccine field. Both use genetic material called mRNA, or messenger RNA, which tells human cells how to make coronavirus spike proteins.