The Food and Drug Administration (FDA) stated that two older adults who received Pfizer’s respiratory syncytial virus (RSV) vaccine during a clinical trial were subsequently diagnosed with the rare neurological disorder Guillain-Barré syndrome.
Briefing documents (pdf) released on Feb. 24 ahead of this week’s meeting of the Vaccines and Related Biological Products Advisory Committee flagged the two cases of the disorder and stated that 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 stated in the documents. “Therefore, [Guillain-Barré] is being considered an important potential risk.”
Two people in their 60s who received the RSV vaccine were diagnosed with Guillain-Barré syndrome in a phase 3 trial, which involved 20,000 recipients of the vaccine. There were no instances of the neurological disorder 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 the FDA asked Pfizer to conduct a safety study if the RSV vaccine is approved in the spring.
The advisory committee is scheduled to meet on Feb. 28 and discuss RSV vaccines from Pfizer and British drugmaker GSK for adults aged 60 and older.
Pfizer stated in its own briefing document (pdf) that the cases of Guillain-Barré syndrome have possible explanations unrelated to its vaccine.
No safety concerns were identified by Pfizer during the trial and the company stated that it would carry out a safety study on its RSV vaccine if approved.
The FDA’s briefing documents state that Pfizer’s RSV vaccine was 85.7 percent effective at preventing severe illness.
What Is Guillain-Barré Syndrome?
Guillain-Barré 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, although most people make a full recovery in a process that can take several years.
The cause of the disorder is unknown, but about 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-Barré syndrome can include the Zika virus, gastrointestinal and respiratory infections, and 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 about 14,000 people older than 65 each year in the United States.
The CDC estimates that there are roughly 2.2 million symptomatic RSV illnesses each year and about 177,000 hospitalizations.
RSV is about as severe as influenza, which the CDC believes is associated with 12,000 to 43,000 deaths each year in people aged 65 and older.
While there are currently no approved vaccines against RSV, that could change if Pfizer’s and GSK’s vaccines get the nod from the advisory panel.