AstraZeneca vaccine and Covid both linked to increased risk of Guillain-Barré syndrome

However, the peer-reviewed study, which covered 32 million people in England with Scottish data used for comparison, also found an association between the AstraZeneca vaccine and a rare but occasionally life-threatening disorder called Guillain-Barré syndrome (GBS), which affects the nerves.

The rate was nearly four- times higher, however, among Covid patients.

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Symptoms of GBS include numbness, pins and needles, and problems with coordination which tend to start in the hands and feet and spread into the limbs.

In severe cases, people can develop blood clots or difficulty moving, walking, breathing, and swallowing.

The condition is treatable and most people make a full recovery, but some are left with permanent disability and around one in 20 people die.

The findings, published in the journal Nature Medicine, provide the most robust evaluation to date of the association between the virus, vaccines and a range of neurological adverse events.

The study, led by Oxford University with input from researchers at Edinburgh University, covered the period from December 1 to May 31.

During this period, 20 million people in England received a first dose of the Oxford-AstraZeneca vaccine, 12 million received a first dose of the Pfizer-BioNTech vaccine, and two million people tested positive for Covid on a PCR test.

Adverse events following boosters, second doses or in children under 16 were excluded from the evaluation due to limited data, and only complications resulting in hospitalisation were included.

The analysis took into account incidence of neurological complications including encephalitis, Bell's palsy, stroke and Guillain-Barré occurring in the first 28 days after either vaccination or a positive PCR test for Covid.

The scientists stress that adverse events occurring in this 28 day window were not necessarily caused by the infection or vaccines. However, the timing points to evidence of a possible link.

This 'risk period' was then compared against a before and after baseline - excluding the 28 day periods either side of first vaccination or positive test - to gauge the normal frequency of these events.

This enabled the researchers - who are independent of the Oxford vaccine developers - to detect an 'excess', meaning that the frequency of complications was above baseline levels.

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In relation to encephalitis, meningitis and myelitis, they found an excess of 123 cases per 10 million people following Covid infection. No association was found for either vaccine.

For Guillain-Barré syndrome, there was an excess of 38 cases per 10 million people following a first dose of the AstraZeneca vaccine, but that compared to an excess of 145 cases per 10 million people following a positive Covid test.

In relation to haemorrhagic stroke, the researchers detected an excess of 60 cases per 10 million people following the Pfizer vaccine.

However, they noted that this pattern was "not replicated" in the Scottish population dataset, but that this may have been due to the comparatively smaller sample size.

They also detected a doubling in the rate of haemorrhagic stroke in the first seven days after a positive Covid test, but found no overall excess when cases were analysed over the full 28 day period.

Initial vaccine clinical trials were not large enough to be able to detect very rare adverse neurological events – those that happen in less than one person out of 10,000.

Aziz Sheikh, a co-author on the paper and professor of primary care at Edinburgh University, said establishing an association between very rare complications and vaccines is important to help doctors recognise and act on them early.

He said it was the "largest and most robust study internationally to date" to put the AstraZeneca vaccine and Guillain-Barré syndrome into context against other vaccines and Covid.

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However, he stressed that in all cases these neurological adverse events were "so rare that we're having to report them per 10 million".

The findings also clearly emphasise that the risk from infection outweighs that from either vaccine type, said Prof Sheikh. 

He added: "A key strength of this study was that we were able to replicate the analysis in Scotland’s national Covid-19 dataset.

"Overall, this provided strong support to the findings observed in the English dataset, namely that Covid-19 poses a greater risk of neurological adverse events than seen with wither the Oxford-AstraZeneca or Pfizer-BioNTech vaccines.

"This study demonstrates the very considerable power of UK data to investigate very rare safety concerns at pace and at scale.

"As far as we’re aware, no other country has this capability to do this and independently replicate findings."

More than four million people in Scotland have now received a first dose Covid vaccination, including 2.1m who had AstraZeneca.

However, one in 90 people are currently estimated to be carrying Covid infections with confirmed cases plateauing at around 2,500 per day.

Julia Hippisley-Cox, co-author and professor of clinical epidemiology and general practice at Oxford University, said: “We know the Covid-19 vaccines are very effective at reducing risks of severe outcomes from Covid-19 infection.

"Whilst there are some increased risks of very rare neurological complications associated with the Oxford-AstraZeneca vaccine, these are much smaller than the risks associated with Covid infection itself.”