New interim results on the Oxford Covid-19 vaccine trials say the jab is effective against symptomatic disease in 70pc of cases - with vaccine efficacy of 62pc for those given two full doses, and of 90pc in those given a half dose then a full dose.
The results published in the Lancet yesterday are the first full peer-reviewed efficacy results to be published for a Covid-19 vaccine.
It comes as the roll-out of the Pifizer BioNTech vaccine got underway in Northern Ireland and mainland UK this morning.
Ireland has an advance purchase agreement to buy 3.3 million of the Oxford vaccine produced with AstraZeneca.
It has yet to be submitted for regulatory approval.
Yesterday's results state the vaccine was found to be safe, with only three out of 23,745 participants, over a median of 3.4 months, experiencing serious adverse events that were possibly related to the vaccine.
All participants have recovered or are recovering, and remain in the trial.
"The results presented in this report provide the key findings from our first interim analysis," said the study's lead co-author, Dr Merryn Voysey of the University of Oxford.
"The Oxford Covid-19 vaccine uses a chimpanzee adenovirus viral vector that cannot cause disease in humans and expresses the SARS-CoV-2 spike protein.
For the new study, the authors analysed data from 23,745 adults in the UK, Brazil and South Africa (11,730, 10,002, and 2,013 in each country, respectively).
The study pooled the data from these countries providing greater precision for efficacy and safety outcomes than possible in individual trials.
The trial was originally designed to assess a single vaccine dose, but a review of the immune response data found a second dose boosted immune response.