The finding is a boost for Britain’s controversial decision to extend the gap between the first and second doses of the vaccine to 12 weeks.
Oxford University said the findings of the pre-print paper show the jab also had a “substantial effect” on reducing transmission of the disease.
Before these results, little was known about how effective the Covid-19 vaccines were at preventing transmission.
Senior public health officials have warned since the first vaccine was approved that there was no data to indicate what impact it would have on spreading coronavirus.
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.
Oxford academics said: “Vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 post vaccination was 76%, and modelled analysis indicated that protection did not wane during this initial three month period.”
The paper said that 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.
The authors also reported a 67% reduction in transmission after the first dose of the vaccine based on swabs obtained from volunteers in the UK arms of the trial.
The report states that the vaccine “may have a substantial impact on transmission by reducing the number of infected individuals in the population”.
The results, gathered from trials in Britain, Brazil and South Africa, indicated that immune responses were boosted with a longer interval to the second dose among participants aged 18 to 55 years.
Professor Andrew Pollard, chief investigator of the Oxford Vaccine Trial, and study 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.”
Cutting transmission rates by 67% could prove to be the “holy grail” of the global vaccine rollout, according to a leading pharmacologist.
The fact it dramatically cut transmission after just one dose will mean lockdown measures can be lifted sooner, a former chair at the Faculty of Pharmaceutical Medicine said.
Dr Gillies O’Bryan-Tear said the results, which have yet to be peer reviewed, were the first definitive estimate of the impact of vaccination on transmission rates.
“If the effect on transmission is confirmed for the Pfizer vaccine too, this would be a very positive,” he said.
“If these vaccines reduce transmission to the extent reported, it will mean that the easing of social restrictions will be enabled sooner, than if we have to wait for herd immunity – which may never in fact be achieved because of insufficient vaccine population coverage.”
He added: “That would be the holy grail of the global vaccine rollout, and these data bring us one step closer.”
If a vaccine only prevents a patient becoming severely ill, but they are still able to catch and pass on the virus, then everyone needs to have received a jab to be protected.
But if the vaccine also stops someone hosting and spreading the virus, then each vaccinated person also protects others.
But Dr O’Bryan-Tear warned that, as yet, there is little data to show how the researchers calculated the 67% reduction in transmission in vaccinated participants compared to unvaccinated participants.
The paper is currently under review at the Lancet ahead of publication.
Dr O’Bryan-Tear said: “Few data were provided on how this figure was calculated, for example, how many samples it represented.
“We await fuller data and the publication, which will appear in the Lancet shortly.”