The AstraZeneca vaccine is associated with a small but significant increased risk of Guillain-Barré syndrome (GBS), a rare neurological condition, a new analysis has revealed.

Researchers from UCL carried out a population-based study of NHS data in England to track GBS case rates against vaccination rollout and found that following a first dose AstraZeneca vaccine there were 5.8 excess GBS cases per million doses of vaccine.

However, researchers say it remains unclear what the cause of the link is and similar increases in the condition have been linked to other mass vaccination campaigns.

An unusual spike in GBS reports during March and April 2021

GBS is an 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.

The condition is usually reversible but in severe cases it can cause prolonged paralysis involving breathing muscles, require ventilator support and sometimes leave permanent neurological deficits.

During March and April 2021, there was an unusual spike in GBS reports (around 140 cases per month compared to the usual 100), which cooccurred at a similar time to the vaccination roll out.

To identify whether any or all of these cases were linked to vaccination, they linked dates of GBS onset to vaccination receipt data held on the National Immunisation Management System in England for every individual.

The analysis revealed that 20% (n=198) of these cases occurred within six weeks of the first-dose of Covid-19 vaccination. Of these, 176 people had had an AstraZeneca vaccination, 21 Pfizer, and 1 (one) Moderna. Only 23 GBS cases were reported within six weeks of any second vaccine dose.

Overall, following a first dose 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-dose Pfizer and Moderna and second-dose of any vaccination showed no excess GBS risk.

Similar increases in GBS have been linked to other mass vaccination campaigns

Commenting on the figures, lead author Professor Michael Lunn (UCL Queen Square Institute of Neurology) said: “Higher numbers of cases of GBS are seen in the period of two to four weeks after vaccination. A peak of cases observed around 24 days following a first dose.

“First doses of AstraZeneca vaccine account for the majority or all of this increase. A similar pattern is not seen with the other vaccines or following a second dose of any vaccine.”

However, the authors note that the small numbers of GBS cases observed appear similar to increases previously seen in other mass vaccination campaigns. 

For example, during the 1976 swine flu vaccination campaign in the USA, there was a small increase in GBS associated with what was a novel flu jab. This correlation halted the vaccination campaign, although subsequent statistical analysis found the risk of a link to be lower than initially thought.

Unclear what causes the small rise in cases

Professor Lunn says it is unclear why a vaccine may cause these very small rises in GBS, but speculates: “It may be that a non-specific immune activation in susceptible individuals occurs, but if that were the case similar risks might apply to all vaccine types.

“It is therefore logical to suggest that the simian adenovirus vector, often used to develop vaccines, including AstraZeneca’s, may account for the increased risk.”

He says further study will now be necessary to determine whether the two are linked.