The European Medicines Agency (EMA) said a causal relationship between GBS and the AstraZeneca shot, known as Vaxzevria, was a “at least a reasonable possibiliy.
It came after 833 cases of GBS were reported out of 592 million doses of the vaccine given worldwide by July 31.
The EMA has categorised the side-effect as “very rare”, the lowest frequency of side-effect category it has, and has emphasised that the benefits of the shot outweigh the risks.
The UK government has updated its own guidance on the GOV.UK, stating: “Since all adults in the UK have been offered a coronavirus (Covid-19) vaccine, a certain number of cases of GBS would be expected following vaccination.
“Cases following Covid-19 vaccination have been reported in the UK and internationally and investigations are ongoing to establish if the rate of reported cases is above what would be expected in the population and whether there is evidence of an association with these vaccines.
“GBS cases have been reported following other vaccination programmes, with much research focusing on GBS and the seasonal flu vaccine.
It added: “The data has been variable from season-to-season, but if there has been an increased risk, it has been in the range of 1 to 2 additional GBS cases per million flu vaccine doses administered.
“The evidence also suggests that an individual is more likely to get GBS after having flu than after the vaccination, with flu also causing severe illness and death in some cases.”
The US Food and Drug Administration has added a warning over Guillain-Barré syndrome as a possible side-effect of Johnson & Johnson’s shot.
Both vaccines use viral vector technology, and have also been associated with rare blood clots.
Additional reporting by the Reuters news agency.