FDA Adds Guillain-Barre Syndrome Warning to Two RSV Vaccines

The FDA issued a safety communication that warns about possible Guillain-Barré syndrome (GBS) with respiratory syncytial virus (RSV) vaccines manufactured by Pfizer (Abrysvo) and GSK (Arexvy).

The prescribing information for each vaccine has been revised to say that a postmarketing observational study suggested an increased risk of GBS in the 42 days following vaccination.

GBS is a rare, acquired demyelinating polyneuropathy that causes muscle weakness, often beginning in the lower extremities and ascending over time with loss of reflexes. It sometimes leads to paralysis.

Before the RSV vaccines were approved, the FDA flagged potential GBS risks with the vaccines. After approval, the CDC continued to monitor for GBS risk.

FDA conducted a postmarketing observational study that assessed the risk of GBS following vaccination with the two companies’ RSV prefusion F protein vaccines using Medicare claims data from May 2023 to July 2024. Based on the FDA’s review of clinical trial data, reports to the Vaccine Adverse Event Reporting System (VAERS), and the postmarketing study, the agency determined that the overall body of evidence suggested increased risks of GBS with Abrysvo and Arexvy, but that available evidence was not enough to establish a causal relationship.

The postmarketing study was a self-controlled case series analysis using risk windows of 1 to 42 days post-vaccination and control windows of 43 to 90 days post-vaccination. The claims data suggested an increased risk of GBS during the 42 days following vaccination, with an estimated nine excess cases of GBS per million doses of Abrysvo, and an estimated seven excess cases of GBS per million doses of Arexvy administered to people 65 years of age and older.

“Background risks of GBS in study populations influence excess GBS case estimates and may differ between studies and analyses within a study, precluding direct comparisons of excess GBS case estimates from other vaccine studies or populations,” the FDA noted.

The CDC recommends a single dose of any FDA-approved RSV vaccine for all adults ages 75 years and older, as well as for 60- to 74-year-olds who are at increased risk of severe illness from the virus. A third approved product, Moderna’s mRNA vaccine (mResvia), was not included in the safety warning.

Abrysvo and Arexvy were initially approved in May 2023 to prevent lower respiratory tract disease (LRTD) caused by RSV in people ages 60 and older. Since then, Abrysvo has been approved to prevent LRTD caused by RSV in people 18 to 59 years who are at increased risk for LRTD caused by RSV, and to immunize pregnant women to prevent LRTD caused by RSV in infants.

Arexvy also has been subsequently indicated for people ages 50 through 59 who are at increased risk for LRTD caused by RSV.

As part of the vaccine approval process, the FDA required Pfizer to conduct a study to evaluate the risk of GBS among adults vaccinated with Abrysvo in the U.S. in all age groups for which the vaccine was approved. The agency requested a similar study to evaluate GBS risk in adults ages 50 and older vaccinated with Arexvy.

The FDA further determined that the benefits of vaccination with the two RSV vaccines continue to outweigh their risks.