Chronic Inflammatory Demyelinating Polyneuropathy (CIDP); This literature case was received on 22-Feb-2021 and concerned an 89-year-old male patient.   The patient had no relevant medical history. He 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. The patient's concomitant medications were not reported. There was no family history of neuropathy.  On an unspecified date, the patient was vaccinated with influenza virus vaccine polyvalent (brand not specified; dose, route of administration, anatomical location and indication: not reported). The batch number was not reported.   On an unspecified date, two days after receiving influenza virus vaccine polyvalent, the patient developed tingling in the hands, followed by progressive weakness, which progressed over two weeks, in his arms and legs.  On an unknown date, the patient started five-day course treatment with intravenous immune globulin (IVIG) over four weeks.   On an unknown date, patient improved.  On an unspecified date, a month later, the patient experienced more gradual worsening of weakness over six weeks. The patient was unable to walk or stand, even with support.   On an unknown date, the patient 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. Examination showed marked weakness in proximal and distal muscles of all extremities, with absent muscle stretch reflexes. The patient's tendon reflexes were all absent. Vibration was diminished in the toes, and other sensations were intact. Electrodiagnostic findings for median motor were: right (rt) distal motor latency (DL) (ms): 9.3, rt: amplitude (Amp) (mV): 1.6, rt: conduction velocity (CV) (m/s): 24, 26 and rt: F wave latency (ms): 54; for peroneal motor: rt: DL (ms): 6.7, rt: Amp (mV): 0.4, rt: CV (m/s): 31, 31 and rt: F wave latency (ms): No response (NR); for tibial motor: rt: DL (ms): 8.3, rt: Amp (mV): 0.7, rt: CV (m/s): 26, rt: F wave latency (ms): NR and for ulnar motor: rt: DL (ms): 4.3, rt: Amp (mV): 3.5, rt: (CV) (m/s): 43, 37, 43, rt: F wave latency (ms): 47. Sural amplitude for left (lt) and rt: NR, median sensory amplitude was: NR and ulnar sensory amplitude was 2.6 mV. Needle Electromyography (EMG) showed fibrillation potentials and loss of motor unit potentials in the distal upper and lower extremity muscles. Electrodiagnostic studies showed widespread abnormalities, indicating a demyelinating polyneuropathy. 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. The cerebral spinal fluid (CSF) analysis revealed elevated protein (127,3 mg/dl), with normal glucose and cell count. Other laboratory and imaging studies were normal.   On an unknown date, the patient was treated monthly with IVIG at dose of 1 g/kg and oral Mycophenalate (mycophenolate mofetil) at dose of 1000 mg, twice a day (bid).  On an unknown date, after six months IVIG was discontinued.   On an unspecified date, at the 16-month follow-up, the patient showed normal gait and only mild weakness of intrinsic hand muscles (grade 4 Medical Research Council (MRC)).  At the time of initial reporting, the patient was recovering from event.   The relationship between immunization, particularly influenza vaccination, and the development of acquired demyelinating neuropathies has 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. Patients, who were 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. 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 had 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.  The event of chronic inflammatory demyelinating polyneuropathy was considered to be medically significant by a Physician within Seqirus' Pharmacovigilance and Risk Management Department.   This case is linked to cases 202101473 and 202101475 (the same literature article).  Company comment: A 89-year-old male 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, electrodiagnostic studies showed widespread abnormalities, indicating a demyelinating polyneuropathy. Information regarding the concomitant medications have not been provided. 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 were 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 89-year-old male 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, electrodiagnostic studies showed widespread abnormalities, indicating a demyelinating polyneuropathy. Information regarding the concomitant medications have not been provided. Based on plausible time relationship, causality for the reported event cannot be totally excluded and is assessed as possibly related.