German authorities have advised that the Oxford/AstraZeneca jab should not be given to those aged 65 or above due to "insufficient data" over its efficacy for older people.
The German announcement comes as the EU is in dispute with the manufacturers over a shortage of vaccines on the continent.
AstraZeneca, which is based in the UK has said production issues at its Europe-based plants mean it will be unable to deliver the promised number of doses to the bloc.
The European Medicines Agency is to decide on Friday whether to approve the vaccine for use across the EU.
Writing in their Lancet study released last year, researchers said there were not enough Covid cases among older adults at the time to explore just how well the vaccine protected them against the disease.
“Vaccine efficacy in older age groups could not be assessed but will be determined, if sufficient data is available, in a future analysis after more cases have accrued,” the team wrote.
The UK has been using the AstraZeneca vaccine in its mass immunisation programme for weeks now, and public health officials say it is safe and provides "high levels of protection".
The UK’s Medicines and Healthcare products Regulatory Agency itself has said there is nothing to suggest a lack of protection in over 65s while Oxford has said early figures were promising: “Preliminary efficacy data in older adults supports the importance of this vaccine for use in this population.”
UK Prime Minister Boris Johnson said he was not worried by the German recommendation, saying British health authorities had made clear the AstraZeneca vaccine was "effective across all age groups".
Prof Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine said: "There is no reason at all for anyone in the UK or elsewhere to think that this Oxford/AstraZeneca vaccine is ineffective at any age."
The speed at which the EU's vaccination programme has been rolled out has been criticised in recent weeks.
There are shortages due to delays in shipments of both the Pfizer-BioNTech and Moderna vaccines, which are the only ones currently approved for use in the block.
The EU signed a deal with AstraZeneca in August for 300 million doses, with an option for 100 million more, but the UK-Swedish company has reported production delays at plants in the Netherlands and Belgium.
The EU and AstraZeneca have said that they will work together to resolve the problems.