The U.S. Food and Drug Administration (FDA) has mandated updates to the safety labeling for Pfizer Inc's (NYSE:PFE) Abrysvo and GSK plc's (NYSE:GSK) Arexvy respiratory syncytial virus (RSV) vaccines.
New warnings in the prescribing information highlight a potential increased risk of Guillain-Barré syndrome (GBS), a rare neurological disorder, within 42 days of vaccination based on findings from a postmarketing observational study.
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GBS, a condition in which the immune system attacks nerve cells, can lead to muscle weakness and, in severe cases, paralysis.
According to the FDA, while the evidence suggests an elevated risk, it is insufficient to confirm a definitive causal link between the vaccines and the disorder.
This determination is based on data from clinical trials, adverse event reports, and an observational study conducted among Medicare beneficiaries aged 65 and older.
The study, conducted between May 2023 and July 2024, utilized Medicare claims to identify cases of hospitalized GBS in individuals vaccinated with Abrysvo or Arexvy.
Using a self-controlled case series analysis, researchers compared risk windows of 1-42 days post-vaccination with control windows of 43-90 days. The findings estimate nine excess GBS cases per million doses of Abrysvo and seven per million doses of Arexvy administered to individuals aged 65 and older.
These risks reflect excess cases compared to background rates of GBS in the studied population. However, variations in background risks and analytical methods make direct comparisons to other studies challenging. The FDA emphasized that while increased risks were observed, further investigation is needed to fully understand the connection between the vaccines and GBS.
U.S. sales of RSV vaccines have declined after regulators narrowed the age group eligible for the shots and determined they would be a one-time vaccination for now.
For Pfizer and GSK, RSV vaccines are critical as they seek to build new revenue streams before facing generic competition for their top-selling drugs.
Price Action: GSK stock is down 1.55% at $33.56, and PFE stock is down 0.18% at $27.08 during the premarket session at last check Wednesday.
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