Two people who took part in a clinical study and received Pfizer’s respiratory syncytial virus (RSV) vaccine were subsequently diagnosed with a rare neurological condition, according to a briefing document released Friday. Two adults in their 60s were confirmed to have Guillain-Barre syndrome after receiving the vaccine as part of Pfizer’s Phase 3 clinical trial for the shot, which has not yet been approved. Nearly 20,000 people got the shot for the trial. The Food and Drug Administration has flagged the two cases as a cause for concern and called on Pfizer to conduct a safety study if the vaccine 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. … Therefore, [Guillain-Barre] is being considered an important potential risk,” the FDA said. Guillain-Barre causes the immune system to attack its own nerves, sometimes causing paralysis. Pfizer has said it will conduct a safety study but also noted that there may be other explanations for the two Guillain-Barre cases, describing the shot as “well-tolerated and safe vaccine, with a benefit-to-risk ratio that is favorable.”