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The condition isn't considered a side effect of the vaccines, the staff wrote, but they plan to track the data on vaccine recipients to spot any potential cases.
The report said 4 of 30,000 participants in the Moderna clinical trial had Bell's palsy, including 3 participants who received the vaccine instead of the placebo. Similarly, 4 out of 43,000 participants in the Pfizer clinical trial had Bell's palsy, and all 4 received the vaccine.
The paralysis occurred between 22 days and 32 days after the shot, the FDA staff said. Two of the Bell's palsy cases in the Moderna trial have resolved. The staff has endorsed both of the COVID-19 vaccines and said there's not enough data to show whether the cases were tied directly to the vaccines, according to CNBC.
"There were no other notable patterns or numerical imbalances between treatment groups for specific categories of adverse events, including other neurologic, neuro-inflammatory, and thrombotic events, that would suggest a causal relationship to the Moderna COVID-19 vaccine," the staff wrote.
Importantly, the rate of Bell's palsy in the clinical trials is lower than the overall rate in the general population, they wrote. About 35 per 100,000 people get Bell's palsy in the U.S. each year, according to the National Organization for Rare Disorders, and about 40,000 Americans are diagnosed annually.
The exact cause of the condition isn't known, but it's typically associated with a viral infection, immune disorder or inflammation in the nerve that controls facial muscles, NORD said.
"It's a relatively benign condition," Anthony Geraci, MD, the director of neuromuscular medicine at Northwell Health in New York, told USA Today.
Geraci sees at least two patients per month with Bell's palsy, and they tend to recover in several weeks, he said. Severe or permanent conditions are extremely rare, he added, and encouraged people to not let the report prevent them from getting a COVID-19 vaccine.
"It's a cautionary tale that should not mitigate the larger good that both individuals and society are going to derive from all of these vaccines," he said.
The CDC said last week that it would monitor vaccine recipients for Bell's palsy but said the COVID-19 vaccines don't appear to cause the condition, according to CNBC. The FDA will also track data on facial paralysis cases as vaccines are administered to patients.
"Our working hypothesis is this just was an imbalance in background rates like we've seen in other trials, but we'll make sure that we're going to actually query for that just to bring that question to close," Peter Marks, MD, director of the FDA's Center for Biologics Evaluation and Research, said in an interview with JAMA on Monday.
Some anti-vaccination groups have used the Bell's palsy cases to question the safety of the vaccines, Reuters reported. But the FDA and CDC, as well as independent experts, have all said there is no reason to be alarmed by the handful of cases.
FDA, "FDA Briefing Document, Vaccines and Related Biological Products Advisory Committee Meeting, December 17, 2020."
CNBC, "FDA staff recommends watching for Bell's palsy in Moderna and Pfizer vaccine recipients."
National Organization for Rare Disorders, "Bell's palsy."
USA Today, "COVID-19 vaccine trials report cases of brief facial paralysis. That's not as scary as it sounds."
CNBC, "CDC will monitor for Bell's palsy among Pfizer vaccine recipients, but sees no causal relationship."
JAMA, "Coronavirus Vaccine FDA Update, posted December 14, 2020."
Reuters: "Fact check: Photo does not show three recipients of Pfizer's COVID-19 vaccine that developed Bell's palsy"
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Cite this: FDA: Track Vaccine Recipients for Facial Paralysis - Medscape - Dec 16, 2020.