The World Health Organisation (WHO) has backed the use of the Oxford/AstraZeneca Covid-19 vaccine in all adults, and has expressed confidence in its effectiveness at tackling variants of the virus.

WHO scientists have issued interim recommendations on the vaccine, saying that the jab could be given people aged 18 and above “without an upper age limit”.

It comes after controversy surrounding the jag led a number of countries to opt not to give the jab to those over the age of 65.

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The WHO’s Strategic Advisory Group of Experts on Immunisation – also known as Sage but not to be confused with the UK’s scientific advisory group with the same name – also said that the jab should be given in two doses – with the second dose delivered between four and 12 weeks after the first.

But Dr Alejandro Cravioto, chairman of the advisory group, told a press briefing that the window of eight to 12 weeks was “the best time” to give a second dose because the delay produces a “much better immune response”.

He said: “We had a long review of the evidence, talked to the experts and the people who are directly involved with a trial.

“Based on the current evidence, Sage recommends that these vaccines should be administered in two doses of half a millilitre each with an interval of between four and 12 weeks between the first and the second dose.”

He acknowledged that there is limited data from clinical trials on over-65s but said results on efficacy for this age group “had a wide confidence interval”.

He added: “And therefore we feel that the response of this group cannot be any different to groups that are of a younger age.

“Since we have identified people over 65 was one of our priority groups in the prioritisation roadmap… looking at the safety and immunogenicity data… we recommend for the vaccine to be used in people 18 years and above, without an upper age limit.

“That means people over 65 years of age should be given the vaccination.”

Dr Katherine O’Brien, director of the department of immunisation, vaccines and biologicals with the WHO, said modelling data had revealed that even when efficacy of the vaccine dropped to as low as 10% “it’s still the right thing to do to immunise older adults with a low efficacy vaccine because of the high risk of severe disease and mortality in that age group”.

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Asked about places which had chosen not to administer the vaccine to the over-65s, she said some countries might have a choice of vaccines to use.

But she added: “There’s no reason, especially because the over-65s are at the very highest risk of severe disease and death, that a product that has significant efficacy – it should go ahead and be used.

“That is the priority group and so there’s no reason to constrain the AZ (AstraZeneca) product from a general recommendation in the view of Sage and adopted by WHO for an age criteria, while we’re also learning and accruing more evidence.”

The WHO is also recommending the use of the vaccine even if there are variants present in a country.

This comes after a small study in South Africa suggested the Oxford/AstraZeneca vaccine was not effective against mild illness caused by the variant discovered there.

Dr Cravioto said: “Even if there is a reduction in the possibility of this vaccine having a full impact in its protection capacity especially against severe disease, there is no reason not to recommend its use even in countries that have the circulation of the variant.”

On variants, Dr O’Brien said there is “a plausible expectation that the vaccine will have efficacy against severe disease, albeit we don’t have the evidence in hand”.

Professor Sarah Gilbert, chief investigator on the Oxford vaccine trial, said the WHO’s recommendation “paves the way to more widespread use of the vaccine to protect people against Covid-19 and gain control of the pandemic”.

Her Oxford colleague Professor Andrew Pollard, hailed it as an “important milestone in extending access to the Oxford-AZ vaccine to all corners of the world”.

The jab is easier to transport than some of the other Covid-19 jabs because it does not need to be stored at ultra-low temperatures.

Asked about when the emergency use listing for the vaccine might be made, Dr O’Brien said she expected a decision by mid-February following meetings this Friday and Monday.

The WHO also said pregnant women should get the jab “if the benefit of vaccination to the pregnant woman outweighs the potential vaccine risks”, including if the person is in a high-risk group such as being a health worker.

Meanwhile, the University of Oxford and pharmaceutical company AstraZeneca have said that they will provide the vaccine on a not-for-profit basis for the duration of the pandemic across the world, and “in perpetuity” to low and middle-income countries.

Dr Soumya Swaminathan, the WHO’s chief scientist, said the guidance was an “important milestone”.

She added: “These guidelines that are being released today are very, very important, because the Oxford/AstraZeneca vaccine is one of the main vaccines at this point in the Covax facility – it’s going to be procured in hundreds of millions of doses, and distributed around the world.

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“And so many countries will be receiving their first tranches of the vaccine from the Covax facility later in February.

“This is one of those vaccines which can be stored in ordinary refrigerator temperatures, and therefore it’s going to be very useful for a large number of countries with different situations on the ground so logistically, it’s easier to use.”

Shares in the London-listed company AstraZeneca dropped heavily after the announcement, losing 1.3% of their value and wiping around £1.2 billion off its stock market valuation in just half an hour.

The pharmaceutical giant is producing the vaccine on a non-profit basis, so does not stand to gain from the rollout.