GSK and Pfizer's respiratory syncytial virus (RSV) vaccines will carry warnings that they can increase the risk of developing a rare neurological disorder, the U.S. Food Drug Administration said on Tuesday.
The regulator conducted a postmarketing trial which suggested increased risk of Guillain-Barré syndrome (GBS) 42 days following vaccination.
However, the available evidence was insufficient to establish a causal relationship, the FDA said.
The risks flagged in the prescribing information of GSK's Arexvy and Pfizer's Abrysvo were not the regulator's strictest "boxed" warnings.
GBS is a rare disorder in which the body's immune system damages nerve cells, causing muscle weakness and sometimes paralysis. RSV, which typically causes cold-like symptoms, is a leading cause of pneumonia in toddlers and older adults.
In Arexvy's clinical trial, a participant had developed GBS after receiving the vaccine, while in Abrysvo's trial, one participant developed the disease and another got a variant of it.
Last year, advisers to the U.S. Centers for Disease Control and Prevention had postponed endorsing Arexvy's use in the 50-59 age group. They had also flagged the risk of GBS.