The World Health Organization recommends using the vaccine developed by the University of Oxford and AstraZeneca even in countries tackling new variants of coronavirus.
Some new forms of the virus appear to make vaccines less effective.
The WHO also says the vaccine can be used in people aged over 65, which some countries have advised against.
Spacing out the two doses, as is happening in the UK, makes the vaccine more effective, it advises.
The Oxford vaccine is seen as the “vaccine for the world” as it is cheap, can be mass produced and is stored in a standard fridge.
However, it has attracted controversy about its effectiveness against new variants, whether it should be used in the elderly and how far apart the doses should be given, due to a lack of data.
The WHO’s Strategic Advisory Group of Experts on Immunization, known as Sage, has been scrutinising evidence from vaccine trials.
Its interim recommendations say the vaccine is 63% effective overall.
However, early data from trials in South Africa showed the vaccine was offering “minimal protection” against mild and moderate disease in young people.
The WHO’s director of immunisation, Dr Katherine O’Brien, said the South African study was “inconclusive” and it was “plausible” the vaccine would still prevent severe disease.
A variant in the country has acquired mutations that seem to help it evade immunity from vaccines and from previous infections.
However Oxford scientists still expect their vaccine to prevent people from becoming seriously ill with Covid-19 and needing hospital treatment.
“There is no reason not to recommend its use even in countries that have circulation of the variant,” said Dr Alejandro Cravioto, the chairman of WHO’s Sage.
There has been criticism about a lack of data on the effectiveness of the vaccine in the elderly with some countries, including France and Germany, advising against using it in the over 65s.
The WHO said even though there was a small number of over 65s in the trials, other studies showed older people had a nearly identical immune response to younger adults so the vaccine should be used.
The scientific advisers also said giving two doses eight-12 weeks apart increased the vaccine’s effectiveness and provided greater protection.
Initially, the WHO had recommended a gap of up to six weeks between doses, only in exceptional circumstances.
Prof Sarah Gilbert, the chief investigator on the Oxford vaccine trial, said: “It is excellent news that the WHO has recommended use of the Sars CoV-2 vaccine first produced in Oxford.
“This decision paves the way to more widespread use of the vaccine to protect people against Covid-19 and gain control of the pandemic.”
Boris Johnson, the UK Prime Minister, said he welcomed the WHO’s support for the Oxford-AstraZeneca vaccine, and for the longer interval between doses.
In the UK, more than 13 million people have now received a vaccine to protect against Covid-19. On Wednesday, another 1,001 deaths were reported within 28 days of a positive test for coronavirus.
Globally, there have been more than 2.3 million deaths with the disease during the pandemic.