What Causes Guillain-Barre Syndrome?

The exact cause of Guillain-Barre syndrome (GBS) is unknown. It happens when the immune system mistakenly attacks the peripheral nerves, the nerves outside the brain and spinal cord. Most cases begin days or weeks after an infection, such as a stomach bug or flu. About 70% of cases follow an infection.

GBS isn't contagious, and it isn't generally considered inherited.

What happens in the body

The immune system normally attacks bacteria and viruses. In GBS it attacks the nerves instead. One explanation is that some molecules on bacteria and viruses look like molecules on nerve cells, so the immune response aimed at the infection also targets the nerves. This is known as molecular mimicry.

The damage depends on the type of GBS:

• Acute inflammatory demyelinating polyradiculoneuropathy (AIDP): the immune system damages the myelin sheath, the insulating layer around nerves, which slows signals. This is the most common form in North America and Europe.
• Acute motor axonal neuropathy (AMAN) and acute motor-sensory axonal neuropathy (AMSAN): the immune system may damage the nerve fibres themselves.
• Miller Fisher syndrome: this variant mainly affects the nerves of the head and is linked to particular antibodies.

It isn't known why most people recover from these infections normally while a few go on to develop GBS.

Infections that can trigger GBS

Symptoms usually start about 10 days after the trigger. About two-thirds of people had symptoms of an infection in the six weeks before.

Campylobacter jejuni is one of the most common triggers. This bacterium causes gastroenteritis, often from undercooked poultry. According to the US Centers for Disease Control and Prevention (CDC):

• About 1 in 1,000 people with a Campylobacter infection develop GBS
• At least 1 in 20 people with GBS recently had a Campylobacter infection, and some studies put this as high as 8 in 20

Other infections linked to GBS include:

• Flu (influenza)
• Cytomegalovirus and Epstein-Barr virus
• Mycoplasma pneumoniae
• Hepatitis A, B, C and E
• HIV
• Zika virus, dengue and chikungunya

COVID-19: some countries reported more GBS after COVID-19 infection, but the link is described as controversial.

Can vaccines cause GBS?

Very rarely, yes, but the risk is extremely low. People are much more likely to develop GBS after an infection such as flu than after a flu vaccine.

• Flu vaccines: the 1976 swine flu vaccine carried the highest known risk, and today's flu vaccines are linked to a much smaller one. See our page on GBS and the flu shot.
• COVID-19 vaccines: the risk may be raised with the adenovirus-vector vaccines (Janssen and AstraZeneca). No increased risk is noted for the mRNA vaccines.
• RSV vaccine: the NHS notes very rare cases in older adults.
• Others: a meningococcal vaccine may carry a minimal increase in risk, and the risk from HPV, hepatitis B and shingles vaccines is debated.

The consensus in the medical review we used is that the benefits of vaccines outweigh the risk of GBS. If you have had GBS, ask your specialist about future vaccines.

Other triggers

GBS occasionally follows something other than an infection:

• Surgery
• Trauma
• Autoimmune disease, such as lupus
• Some medicines, including immune checkpoint inhibitors (a type of cancer treatment), TNF-alpha inhibitors and type I interferons
• Cancers such as lymphoma, including Hodgkin lymphoma

Who is at risk?

Anyone can get GBS, at any age. It is more common in adults over 50 and slightly more common in men.

Is GBS contagious or genetic?

GBS itself can't be passed from person to person, although the infection that triggered it might be. Outbreaks of GBS are very rare, and usually happen during an outbreak of Campylobacter. See is GBS contagious?

GBS isn't considered inherited. Normal variations in certain immune-related genes may raise the risk, but more research is needed. See is GBS genetic?

Is the damage permanent?

Most people recover fully, even after severe illness, but recovery can take years. Some people have lasting weakness, numbness, fatigue or pain, and some remain permanently disabled. See life expectancy and recovery.

When to get help

Tingling or weakness in the days or weeks after an infection isn't always GBS, but it needs checking. In the UK, ask for an urgent GP appointment or contact NHS 111. Get emergency help if tingling or weakness spreads quickly or moves up your body, or if you have trouble breathing. See symptoms of GBS and how it is treated.