It comes after a number of countries have opted not to give the jab to those over the age of 65.
Scientists advising the World Health Organisation (WHO) have recommended the use of the Oxford/AstraZeneca Covid-19 vaccine in all adults.
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”.
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.
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 added: “In the case of the data coming from clinical trials, we have seen that there was a small participation of people over 65 years of age.
“However, the results of the efficacy estimate for persons up to 65 and older, had a wide confidence interval. 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.”
The Oxford jab is an important vaccine for the global supply.
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.
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.
“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.