Oxford University and AstraZeneca's COVID-19 vaccine showed in a study it had 76 percent efficacy against symptomatic infection for three months after a single dose, which increased if the second shot is delayed, backing Britain's vaccine rollout policy.

The findings of the pre-print paper, which had not been peer-reviewed, supported Britain's decision to extend the interval between initial and booster doses of the shot to 12 weeks, Oxford said on Tuesday. 

An NHS nurse prepares for the next group of people to receive the AstraZeneca/Oxford Covid-19 vaccination on February 1, in the vaccination centre at the Royal Cornwall Showground Wadebridge, England. Photo: VCG

However, the new study did not address concerns about a lack of data on efficacy among the oldest, who the British government has given highest priority in vaccine rollout.

Andrew Pollard, chief investigator of the Oxford vaccine trial, said that the data showed the 12-week interval between doses was "the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose."

Britain has decided to vaccinate as many people as possible as quickly as possible by lengthening the amount of time between initial shots and booster shots.

AstraZeneca's research chief has said 8-12 weeks between doses seems to be the "sweet spot" for efficacy, contrasting with US drugmaker Pfizer, which has warned that the vaccine it has developed with Germany's BionTech was not trialed with such an interval.

The results for the AstraZeneca/Oxford vaccine showed that immune responses were boosted with a longer interval to the second dose among participants aged 18 to 55 years.