Washington, Feb 27 (IANS): A new vaccine for RSV for elderly may potentially increase the risk for Guillain-Barre syndrome (GBS), a rare neurologic disorder, the US Food and Drug Administration (FDA) has said.

GBS is often followed by a bacterial or viral infection that wreaks havoc on the nerves. In people afflicted with GBS, the immune system mistakenly attacks healthy nerves outside the brain and spinal cord, leading to weakness and, sometimes, paralysis.

Currently, there is no approved vaccine or RSV -- a highly contagious virus that causes flu-like illness in people of all ages that increases in severity with age. After decades of trials, the first ever RSV vaccine is now slated for approval by August.

Two adults in their 60s who were part of 20,000 participants of Pfizer's Phase 3 clinical trial for RSV have been diagnosed with GBS. The FDA has, thus, asked Pfizer to conduct a safety study if the shot is approved, according to the recently published agency briefing documents.

While one person's illness had completely resolved after three months, and another was improving after six months. There were no cases among people who didn't receive the shot, the FDA document said.

Pfizer noted that the said cases may have other possible explanations -- one patient had a heart attack and the other had symptoms of an upper respiratory infection. Further, the company stated that its shot is a "well-tolerated and safe vaccine, with a benefit-to-risk ratio that is favourable".

But the FDA said that given the incidence of Guillain-Barre syndrome in the general population is about 3 cases per 100,000 people annually, Pfizer should view the incidents as an important potential risk in its safety surveillance.

Pfizer said it will conduct a safety study on Guillain-Barre syndrome if its RSV shot is approved.

"Given the temporal association and biological plausibility, FDA agrees with the assessments of the investigators that these events were possibly related to study vaccine. a Therefore, (Guillain-Barre) is being considered an important potential risk," the FDA documents said.

The Pfizer vaccine was 66.7 per cent effective at preventing moderated lower respiratory tract illness with two or more symptoms and 85.7 per cent effective at preventing severe illness, according to the FDA documents.

GSKa¿s vaccine against RSV also showed a possible risk of GBS, but the company said there was insufficient evidence to confirm a diagnosis, CNBC.com reported.

GSK's RSV vaccine candidate for older adults was 83.5 per cent effective at preventing severe lower respiratory tract disease.

The company has listed Guillain-Barre as an important potential risk in its safety surveillance plan, according to the FDA. The agency said it will review the plan and make recommendations as needed.