Pfizer's new RSV vaccine for older adults may trigger the potentially deadly Guillain-Barre syndrome, the Food and Drug Administration (FDA) has warned.

In clinical trials, two participants in their 60s developed the condition that can also cause crippling pain, muscle weakness and paralysis.

One of the patients recovered after three months, but the second took six months to return to normal. They were among 20,000 adults who got the experimental shot.

Among the control group who did not get the vaccine, there were no cases of GBS, which has prompted the FDA to ask Pfizer to conduct a safety review before approving the vaccine.

Several pharmaceutical companies are developing a vaccine against respiratory syncytial virus (RSV), a winter respiratory bug that kills up to 10,000 adults over 65 in the US every year. There is currently no shot against the disease available. 

Guillain-Barre sees the immune system go haywire and start to attack nerve cells, triggering symptoms including muscle pain, numbness and pins and needles.

It was also recorded during trials of a rival RSV vaccine made by UK-based pharmaceutical giant GSK. 

The condition has also previously been reported in people who got AstraZeneca's Covid vaccine, which was not approved for use in the US.

One of the patients to develop the syndrome in Pfizer's trial was an American man in his 60s with a history of high blood pressure.

He suffered lower back pain eight days after vaccination and experienced weakness in his legs on the 14th day.