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Briefing documents (pdf) released on Feb. 24 ahead of an upcoming meeting of the Vaccines and Related Biological Products Advisory Committee flagged the two cases of the neurological disorder and said Pfizer’s vaccine poses a potential risk.
“Given the temporal association and biological plausibility, FDA agrees with the assessments of the investigators that these events were possibly related to study vaccine,” the FDA said in the documents. “Therefore, [Guillain-Barre] is being considered an important potential risk.”
Two people in their 60s who received the RSV shot were diagnosed with Guillain-Barre syndrome in a phase three trial, which involved 20,000 recipients of the vaccine. There were no instances of the neurological disorder found in people who received a placebo.
One of the people who experienced the illness made a full recovery after three months and another was showing improvement after half a year.
The briefing documents show that FDA asked Pfizer to conduct a safety study if the RSV vaccine is approved in spring.
The advisory committee is scheduled to meet on Feb. 28 and discuss RSV vaccines from Pfizer and GSK for adults aged 60 and older.
Pfizer said in its own briefing document (pdf) that the cases of Guillain-Barre syndrome have possible explanations unrelated to its vaccine.
No safety concerns were identified by Pfizer during the trial and the company said it would carry out a safety study on its RSV vaccine if approved.
The FDA’s briefing documents say that Pfizer’s RSV vaccine was 85.7 percent effective at preventing severe illness.
What Is Guillain-Barre Syndrome?
Guillain-Barre syndrome is a rare neurological disorder in which the immune system attacks one’s own nerves.
The first symptoms are weakness and tingling in the hands and feet. Severe cases can lead to paralysis or death.
There’s no cure for the syndrome, though most people make a full recovery in a process that can take several years.
The cause of the disorder is unknown but around two-thirds of patients report symptoms of an infection about a month and a half before the onset of the illness.
The infections preceding Guillain-Barre syndrome can include Zika virus, gastrointestinal and respiratory infections, as well as COVID-19.
What Is RSV?
RSV is a virus that causes flu-like symptoms and is highly contagious. Older people tend to have more severe symptoms when afflicted with RSV, which the Centers for Disease Control and Prevention (CDC) estimates (pdf) kills around 14,000 people over 65 each year in the United States.
The CDC estimates that there are around 2.2 million symptomatic RSV illnesses each year and around 177,000 hospitalizations.
RSV is about as severe as influenza, which the CDC estimates is associated with 12,000–43,000 deaths each year in people aged 65 and older.
There are currently no approved vaccines against RSV, but that could change if Pfizer’s and GSK’s shots get the nod from the advisory panel.
By Tom Ozimek