The newspaper said none of the more than 2,000 trial participants had been hospitalised or died.
"However, we have not been able to properly ascertain its effect against severe disease and hospitalisation given that subjects were predominantly young healthy adults," the AstraZeneca spokesman said.
The company said it believed its vaccine could protect against severe disease, given that the neutralising antibody activity was equivalent to that of other COVID-19 vaccines that have demonstrated protection against severe disease.
The trial, which involved 2,026 people of whom half formed the placebo group, has not been peer-reviewed, the FT said.
While thousands of individual changes have arisen as the virus mutates into new variants, only a tiny minority are likely to be important or change the virus in an appreciable way, according to the British Medical Journal.