A study done after the respiratory syncytial virus shots were approved suggests that people given GSK’s Arexvy and Pfizer’s Abrysvo may be at increased risk of Guillain-Barré syndrome for 42 days following immunization, the agency said in a safety communication issued Tuesday. 

The analysis found nine excess cases of Guillain-Barré for every million doses given of Abrysvo, and an estimated seven excess cases per million doses of Arexvy in people who are 65 and older, the FDA said. While the data suggests an increased risk with the vaccines, there isn’t enough evidence to show the shots are causing the condition, the agency said.

GSK and Pfizer didn’t immediately reply to an emailed request for comment.

The condition, where the body’s immune system attacks the patient’s own nerves, causes muscle weakness and can lead to paralysis, according to the FDA. It’s often preceded by an infection and there is no known cure.

The potential risk of respiratory syncytial virus shots has been known, with the Center for Disease Control and Prevention’s vaccine advisory panel discussing the danger last year. The data at the time was mixed, however, and the FDA’s post-marketing study was underway to clarify the issue. 

This isn’t the first time the FDA issued a warning reporting Guillain-Barré syndrome among people who have received a vaccine. In 2021, federal regulators revised the fact sheet for Johnson & Johnson’s Covid-19 shot to warn about a “small possible risk” for the condition.

Before the pandemic, Guillain-Barré syndrome was tied to influenza vaccines, leading some to question the safety of the shots and deepening the barrier to vaccination. The safety of other immunizations that have been debated include those for human papillomavirus and meningococcal disease, though the evidence about them has been limited.