Researchers from the university, which developed the jab with AstraZeneca, said a single dose is 76 per cent effective after three months, while a second dose is 82.4 per cent effective.

This means Covid cases among groups given one and two doses was 76 per cent and 82.4 per cent lower respectively than groups given a placebo.

The study, which has not yet been peer-reviewed, also suggests the vaccine may reduce transmission of the virus by as much as two-thirds.

Researchers analysed UK Covid test swabs which tested positive and found a 67 per cent reduction in positive swabs among those who had been vaccinated with the Oxford-AstraZeneca jab.

The jury’s still out on whether Covid-19 vaccines can stop transmission and not just prevent illness, and the findings could prove a major step in swaying public health advisers to believing they do.

This could pave the way for lockdown restrictions to be lifted much sooner as it would indicate fewer people need to be vaccinated in order to protect the population overall.

The study also supports the strategy of splitting the primer dose and a follow-up booster jab by four to 12 weeks, which has been followed by the UK and a number of other countries.

The Oxford-AstraZeneca jab has started to be rolled out across the UK

Professor Andrew Pollard, chief investigator of the Oxford Vaccine Trial and the study’s co-author, said: ‘These new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA (Medicines and Healthcare products Regulatory Agency) and EMA (European Medicines Agency) to grant the vaccine emergency use authorisation.

‘It also supports the policy recommendation made by the Joint Committee on Vaccination and Immunisation (JCVI) for a 12-week prime-boost interval, as they look for the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose of the vaccine.’