GSK and Pfizer's respiratory syncytial virus vaccines will carry warnings that they can increase the risk of developing a rare neurological disorder, the FDA said.

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.

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 CDC had postponed endorsing Arexvy's use in the 50-59 age group. They had also flagged the risk of GBS.