The Covid-19 vaccine developed by Oxford University and AstraZeneca has 76% efficacy against symptomatic infection for three months after a single dose, with greater effectiveness when a second is given later, a study showed on Tuesday.

Oxford University said the findings of the pre-print paper, which had not been peer-reviewed, supported Britain’s decision to extend the interval between initial and booster doses of the shot to 12 weeks.

Britain has decided to give as many people as possible some protection by lengthening the amount of time between initial shots and booster shots of Covid-19 vaccines.

AstraZeneca has welcomed the move, saying flexibility to extend the time between doses is the best strategy for the shot.

The results, gathered from trials in Britain, Brazil and SA, indicated some protection was given after one shot and immune responses were boosted with a longer interval to the second dose among participants aged 18 to 55 years.

“Vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 after  vaccination was 76%, and modelled analysis indicated protection did not wane during this initial three month period,” Oxford academics said.

The paper said vaccine efficacy was 82.4% with 12 or more weeks to the second dose, compared to 54.9% for those where the booster was given under six weeks after the first dose.