The Johnson and Johnson Covid-19 adenovirus vector vaccine has encountered another problem with rare side effects, specifically Guillain-Barré syndrome (GBS), a rare, neurological, paralysing, autoimmune disorder. The US Food and Drug Administration (FDA) has updated its label for the vaccine to reflect the possibility of increased risk for this rare side effect within 42 days following vaccination.

While the vaccine has not been proven to be a causative effect of the disorder, influenza vaccines and the shingles vaccine have also been linked to an increased risk of contracting the syndrome. As of 30 June, there have been 100 preliminary reports of GBS in the US out of 12.8 million Johnson and Johnson vaccine doses administered, and one confirmed patient death.

On 9 July, AstraZeneca’s Covid-19 vaccine was also recommended to add GBS as a rare side effect by the European Medicines Agency’s (EMA) Pharmacovigilance Risk Assessment Committee (PRAC). There have been 227 cases of GBS out of around 51.4 million doses of the AstraZeneca Covid-19 vaccine administered in the EU and European Economic Area (EEA). There have been no similar GBS side effects identified with Moderna’s and Pfizer/BioNTech’s Covid-19 vaccines.

This news of GBS as a rare side effect follows reports of rare thrombosis with thrombocytopenia syndrome (TTS) side effects for the Johnson and Johnson vaccine. These events have all occurred in women younger than 50 years of age and involve blood clots with low platelets.

Despite these rare side effects, analysis indicates that the potential benefits of the Johnson and Johnson Covid-19 vaccine outweigh the risks, although this should be continually monitored in case Covid-19 variants prove to reduce the vaccine’s effectiveness. For example, preliminary evidence indicates that the AstraZeneca vaccine, another adenovirus vector vaccine, may only be 33% effective against the Delta variant.

GBS is a rare autoimmune neurological disorder in which the immune system attacks the peripheral nervous system, and is often triggered by an antecedent illness such as influenza, Campylobacter jejuni infection, mycoplasma pneumonia, cytomegalovirus infection or Epstein-Barr virus infection. The clinical presentation of GBS is highly variable, resulting in difficulty diagnosing the syndrome. GBS patients progress in weakness, with around 25% requiring artificial ventilation, 20% becoming unable to walk independently after six months, and 2–5% dying. Around 3–5% of patients will experience a relapse at some point in their lifetimes.

The two main treatments for GBS are intravenous immunoglobulin (IVIg) and plasma exchange, which can reduce the severity and shorten the duration of the illness. Few recent advances have been made in treating the syndrome, although some recent experimental studies have shown that complement inhibition combined with IVIg could improve outcomes.