London, A correlation between a first dose of the AstraZeneca vaccine and a small but significant rise in cases of the serious neurological condition Guillain-Barre syndrome (GBS), has been identified.

Researchers at the University College London (UCL) said they are yet to understand the cause of the link. However, they noted that the small numbers of GBS cases observed appear similar to increases previously seen in other mass vaccination campaigns.

Guillain-Barre syndrome (GBS) is a rare but serious autoimmune condition that attacks the peripheral nervous system, typically resulting in numbness, weakness and pain in the limbs and sometimes resulting in paralysis of breathing.

In the UK, Pfizer's Covid-19 vaccine was rolled out in December 2020, followed by AstraZeneca in January 2021, then Moderna in April 2021.

In the study, published in the journal Brain, researchers observed that between January to October 2021, 996 GBS cases were recorded in the UK, but with an unusual spike in GBS reports occurring between March and April 2021.

For these two months there were about 140 cases per month, compared to historical rates of about 100 per month.

The analysis revealed 198 GBS cases (20 per cent of 966) occurred within six weeks of the first-dose Covid-19 vaccination in England, equating to 0.618 cases per 100,000 vaccinations.

Of these, 176 people had had an AstraZeneca vaccination, 21 Pfizer, and one Moderna. Only 23 GBS cases were reported within six weeks of any second vaccine dose.

Overall, following a first dose of AstraZeneca vaccine there were 5.8 excess GBS cases per million doses of vaccine, equating to an absolute total excess between January-July 2021 of between 98-140 cases. First doses of Pfizer and Moderna and second doses of any vaccination showed no excess GBS risk.

"The reason for the association between only AstraZeneca vaccination and GBS is unclear. Covid-19 infection does not have a strong, or possibly any, increased risk of GBS," said lead author Professor Michael Lunn at UCL.

"The lack of increased risk associated with Pfizer vaccination implies that it is unlikely that the Covid-19 spike protein is the causative factor for the increased risk.

"The viral vector used to carry the nucleic acid in the AstraZeneca and similar vaccines may be the reason but this needs further exploration," Lunn said.

In a separate phenotype study, researchers analysed GBS cases reported by clinicians between January 2021 and November 2021.

They found that no specific clinical features, including facial weakness (which has received special attention in medical literature), were associated with vaccination-related GBS compared to non-vaccinated cases, showing how difficult it is to spot vaccination linked cases from among the background cases occurring normally.