About 100 preliminary cases of the syndrome have been reported after the distribution of nearly 13 million doses of the Johnson & Johnson vaccine in the United States. Of these cases, 95 were serious and required hospitalization and there was one death, per the FDA statement.

Guillain-Barré syndrome is an autoimmune disorder that affects the peripheral nervous system. It can cause full-body paralysis and can lead to death, though most people fully recover. Most of the cases in people vaccinated with the Johnson & Johnson shot were reported within 42 days of vaccination and were mostly in men aged 50 and older.

“Although the available evidence suggests an association between the Janssen vaccine and increased risk of GBS, it is insufficient to establish a causal relationship,” FDA said.

The warning is another setback for the shot, which is the only single-dose vaccine approved for use in the United States.

“We strongly support raising awareness of the signs and symptoms of rare events to ensure they can be quickly identified and effectively treated,” Johnson & Johnson said in a statement yesterday. The company reiterated that despite the warning, its vaccine remains important in the effort to create immunity to COVID-19. Nearly all COVID-19 deaths and hospitalizations in the U.S. are among unvaccinated people, the White House said last week.

“A single-shot vaccine that provides this level of protection represents an important tool in the global fight against COVID-19, as we strive to help end this deadly pandemic,” the company said.

What to Know About Guillain-Barré Syndrome

GBS is an autoimmune disorder in which the immune system attacks the nervous system outside the brain and spinal cord. The disorder is rare—there are 300,000-600,000 cases in the U.S. each year. 

Early symptoms include weakness and tingling in the extremities. In mild cases, people may experience brief muscle weakness. In extreme cases, these sensations can develop into full-body paralysis, limiting a person’s ability to breath independently or walk.

Most people, even those who have extreme symptoms, fully recover with only lingering muscle weakness. At six months, about 60-80% of people are able to walk. The mortality rate is between 4% and 7%.

About two-thirds of people who develop GBS do so after experiencing respiratory or gastrointestinal issues. Some countries reported an increase in GBS cases after surges in infections by the Zika virus.

Scientists don’t yet know the exact cause of GBS, but it has been shown to be triggered by various viral infections. P. James Dyck, MD, a neurologist at the Mayo Clinic, says that because of the association between viral infections and GBS, he expected some COVID-19 vaccines might increase the risk of the syndrome as well.

“It’s an autoimmune disease that is often induced by a viral infection. Your body gets confused and there’s an autoimmune attack on nerves,” Dyck tells Verywell. There have been reports of COVID-19 being associated with Guillan-Barré syndrome. So, the expectation was that you would have some association with Guillain-Barré Syndrome and vaccinations for COVID-19."

Various vaccines, including some influenza immunizations and a key shot to prevent shingles, have been linked to GBS. For instance, the U.S. halted its 1976 swine flu vaccination program after reports of GBS among vaccinated people. Researchers estimate the vaccine increased the risk of GBS by just under one case per 100,000 vaccinations, according to a retrospective study in the New England Journal of Medicine.

Last week, the European Medicines Agency’s Pharmacovigilance Risk Assessment Committee recommended a similar warning label for the AstraZeneca COVID-19 vaccine. The U.S. has yet to authorize the vaccine. So far, there is no strong evidence that the vaccine triggers the condition.

“At this stage the available data neither confirms nor rules out a possible association with the vaccine,” the EMA said.

In general, when vaccination is linked with triggering GBS, there are typically only one to two additional GBS cases per million vaccine doses delivered, according to the CDC.

“In the big scheme of things, I personally think trying to protect oneself from COVID-19 and getting a vaccination is certainly the safer option,” Dyck says.

What This Means for the Johnson & Johnson Vaccine

The FDA authorized the Johnson & Johnson vaccine in February. The FDA has now amended the Emergency Use Authorization. The label must now include the warning: “Reports of adverse events following use of the Janssen COVID-19 Vaccine under emergency use authorization suggest an increased risk of Guillain-Barré syndrome during the 42 days following vaccination.”

The warning presents another set-back for the vaccine. In April, regulators placed a temporary pause on the rollout after the shot was linked to reports of a rare blood clot.

So far, neither of the shots that use messenger RNA technology, developed by Pfizer/BioNTech and Moderna, seem to elevate the risk for Guillain-Barré.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.