The UK policy of leaving 12 weeks between doses of the Oxford AstraZeneca coronavirus vaccine boosts the jab’s effectiveness, a new study has found.

A single dose of the vaccine offers 76% protection against Covid-19 from 22 days after vaccination, and this does not wane by the three-month mark, researchers say.

With three months between the two doses, there was an overall efficacy of 81% – compared to 55% for a six-week interval, according to the University of Oxford research published in The Lancet.

But the researchers are urging people to have two doses of the vaccine because they say it is not yet clear how long protection with a single dose of the vaccine may last.

The findings support the policy recommendation made by the Joint Committee on Vaccination and Immunisation (JCVI) for a 12-week interval between doses.

Experts advising the Government said the focus should be on giving at-risk people the first dose of whichever vaccine they receive, rather than providing the required two doses in as short a time as possible.

But some scientists had raised concerns about this approach, suggesting it needed to be tested in order to monitor how changing the dosing regimen impacts vaccine effectiveness.

The new study suggests the interval between doses can be safely extended to three months given the protection a single dose offers, which may allow countries to vaccinate a larger proportion of the population more rapidly.

Study lead author Professor Andrew Pollard, University of Oxford, said: “Vaccine supply is likely to be limited, at least in the short term, and so policy-makers must decide how best to deliver doses to achieve the greatest public health benefit.

“Where there is a limited supply, policies of initially vaccinating more people with a single dose may provide greater immediate population protection than vaccinating half the number of people with two doses.

“In the long term, a second dose should ensure long-lived immunity, and so we encourage everyone who has had their first vaccine to ensure they receive both doses.”

Researchers combined data from trials in the UK, Brazil, and South Africa, including 8,948, 6,753, and 1,477 people, respectively – totalling 17,178 people.

Participants were aged 18 years and over and either received two standard doses of the Oxford vaccine (8,597 participants) or a placebo.

Looking at the interval between two standard doses and its impact on efficacy, participants who were given their doses 12 or more weeks apart had greater protection (81%) than people given their two doses less than six weeks apart (55%).

The results were supported by immune response results in 18-55-year-olds, which found that binding antibody responses were more than two-fold higher in the group having their two vaccines with a longer delay.

Following a single standard vaccine dose, vaccine efficacy from 22 days to three months after vaccination was 76%, researchers found.

Modelling indicated that this protection did not reduce over the three months.

In addition, antibody levels against the coronavirus spike protein remained at similar levels for three months, according to the study.

The authors estimate that a single dose of the vaccine may lead to a 64% reduction in community transmission, and that two doses may reduce cases by 50%.

The effect of two doses appears less than a single dose because there are more asymptomatic cases included in this part of the analysis, and vaccine efficacy against asymptomatic cases is lower.

Professor Pollard said real-world assessments of how the vaccine is working in the population will be needed to confirm this preliminary result.

In other vaccine news, the Pfizer-BioNTech coronavirus vaccine no longer needs to be kept extremely low temperatures, the companies have announced.

The vaccine can be kept at between -15C and -25C  (-13F to 5F) in a normal medical freezer for as long as two weeks, according to new stability data submitted to US regulators.

Current labels for the vaccine state it must be stored in an ultra-cold freezer at temperatures between -80C and -60C (-112F to ‑76F).

The higher temperatures will make the vaccine easier to distribute in the UK and internationally.