Jan 7 (Reuters) - GSK and Pfizer's ( PFE )

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 ( PFE ) 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.