Chronic Inflammatory Demyelinating Polyneuropathy (CIDP); This literature case was received on 22-Feb-2021 and concerned an 84-year-old, female patient.  The aim of this study was to describe three patients who developed chronic inflammatory demyelinating polyneuropathy (CIDP) with rapid onset in close proximity to influenza vaccination followed by relapsing or chronic course. Patients were evaluated at the neuromuscular clinic from 2001 to 2005. The patients underwent detailed history, general and neurological examinations, electrodiagnostic and laboratory studies, including spinal fluid, complete blood count, metabolic panel, serology for autoimmune disorders, Lyme serology, venereal disease research laboratory (VDRL), serum immune fixation, thyroid studies, glucose and HbA1C, and magnetic resonance imaging (MRI) of the brain and spine.  The patient had no relevant medical history. The patient was in good health and had no acute illness or systemic infections. The patient had no diabetes mellitus, history of alcohol abuse, or exposure to any known neurotoxins and had no family history of neuropathy. The patient's concomitant medications were not reported.  On an unknown date, the patient was vaccinated with influenza virus vaccine polyvalent (brand not specified; dose, route of administration and anatomical location: not reported) for influenza vaccination. The batch number was not reported.  On an unknown date, 21 days after receiving influenza virus vaccine polyvalent, the patient experienced severe pain in a band like distribution in the mid thoracic region.  On an unknown date, four days later, the patient experienced progressive weakness and numbness which progressed over six weeks, in her legs and hands. The patient was able to take a few steps with a walker, and she had marked weakness in proximal and distal muscles of upper and lower extremities. Muscle stretch reflexes were absent in the legs and diminished in the arms. Vibration was absent in the feet, but proprioception and pin sensation were intact. Gait was broad based and ataxic.   On an unknown date, the patient started a five-day course of intravenous immunoglobulin (IVIG), but did not improve. The patient was then treated with weekly pulse oral prednisone starting at dose of 150 mg/wk for two months followed by a slow taper to 20 mg/wk.  On unknown date, CSF protein was elevated (136 mg/dl), with normal glucose and cell count. Other laboratory and imaging studies were normal. Electrodiagnostic studies showed evidence of demyelinating polyneuropathy. These values indicated primary demyelination: sural amplitude showed no response, peroneal motor distal latency (DL) was 11 ms (right) and 12.2 ms (left), amplitude (AMP) was 0.5 mV (right) and 0.3 mV (left), and conduction velocity (CV)  in  different segments revealed  21, 22 m/s (right) and 22, 23 m/s (left). Tibial motor DL was 10.7ms (right) and 9.9 ms (left), AMP was 2 mV (right) and 2.3 mV (left), and CV in different segments was 27 m/s (right) and 34 m/s (left) while F wave latency showed no response (NP) (right) and 120 ms (left). Ulnar motor DL revealed 5.1 ms (right and left), amplitude was 5.1 mV (right) and 3.8 mV (left), CV was 37, 34 and 30 m/s (right) and 35, 36 and 38 (left), while F wave latency was 50 (right) and 48 (left). Many nerves showed findings suggestive of primary demyelination, namely temporal dispersion or conduction block, marked reduction of conduction velocity, prolongation or absent F waves, prolonged distal motor latencies, and met the commonly accepted criteria for diagnosis of CIDP. This patient had second relapse and progressive course.   On an unknown date, at 14-months follow-up, the patient was able to ambulate without assistance, and she has mild weakness (grade 4 MRC) in deltoid, intrinsic hand muscles, and hip flexion. Reflexes remain absent in the lower extremities.  On an unknown date, at 16-months follow-up, the patient was able to ambulate independently and had mild weakness in arms.  At the time of initial reporting, the patient was recovering from the event.  The author assessed the event as related to influenza virus vaccine polyvalent. The patient who was otherwise healthy, developed CIDP with rapid onset in close proximity to influenza vaccination, suggesting a plausible relationship between the immunization and the induction of CIDP.  The event of CIDP was considered to be medically significant by a Physician within Seqirus's Pharmacovigilance and Risk Management Department.  The relationship between immunization, particularly influenza vaccination, and the development of acquired demyelinating neuropathies had been discussed in the literature, mostly in the context of Guillain-Barre syndrome (GBS). The plausible relationship between CIDP and vaccination was less clear than between GBS and vaccination. Rapid onset resembling GBS occurs in about 15 percent to 20 percent of cases of CIDP. It was possible that antecedent events such as immunization were more likely in such patients, but that has not been systematically evaluated. In contrast to GBS, which was a monophasic illness, how a single event such as immunization may trigger induction of a relapsing or chronic immune-mediated disorder such as CIDP was not known.   This case is linked to cases 202101473 and 202101474 (the same literature article).  Company comment: A 84-year-old female patient developed chronic inflammatory demyelinating polyradiculoneuropathy after vaccination with the suspect product, influenza virus vaccine polyvalent. Chronology is plausible. The patient was in good health prior to the onset of chronic inflammatory demyelinating polyneuropathy (CIDP) and did not have any acute illness or systemic infections, diabetes mellitus, history of alcohol abuse, or exposure to any known neurotoxins. As reported, the patient had multiple relapses with varying diagnostic findings. More information regarding the concomitant medications. is needed. Based on plausible time relationship, causality for the reported event cannot be totally excluded and is assessed as possibly related.; Reporter's Comments: The patient who was otherwise healthy, developed CIDP with rapid onset in close proximity to influenza vaccination, suggesting a plausible relationship between the immunization and the induction of CIDP.; Sender's Comments: A 84-year-old female patient developed chronic inflammatory demyelinating polyradiculoneuropathy after vaccination with the suspect product, influenza virus vaccine polyvalent. Chronology is plausible. The patient was in good health prior to the onset of chronic inflammatory demyelinating polyneuropathy (CIDP) and did not have any acute illness or systemic infections, diabetes mellitus, history of alcohol abuse, or exposure to any known neurotoxins. As reported, the patient had multiple relapses with varying diagnostic findings. More information regarding the concomitant medications. is needed. Based on plausible time relationship, causality for the reported event cannot be totally excluded and is assessed as possibly related.