developed exacerbated CIDP (or AIDP); developed exacerbated CIDP (or AIDP); This literature-non-study case was reported in a literature article and describes the occurrence of CHRONIC INFLAMMATORY DEMYELINATING POLYRADICULONEUROPATHY (developed exacerbated CIDP (or AIDP)) and GUILLAIN-BARRE SYNDROME (developed exacerbated CIDP (or AIDP)) in an 87-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 vaccination.       The patient's past medical history included Sick sinus syndrome (dual-chamber pacemaker for sick sinus syndrome presented with acute worsening weakness and paresthesia of bilateral lower extremities), Acute inflammatory demyelinating polyradiculoneuropathy in 2017, Bypass surgery and Stent placement. Previously administered products included for Product used for unknown indication: Influenza vaccine (In 2017, shortly after receiving influenza vaccine, he had symptoms, laboratory and and electrophysiological features of GBS/AIDP) in 2017. Past adverse reactions to the above products included Adverse event with Influenza vaccine. Concurrent medical conditions included Coronary artery disease and Chronic inflammatory demyelinating polyneuropathy (received plasmapheresis every 2 weeks with symptom improvement but not normalization.) since 2017.    In 2021, the patient received dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. In 2021, the patient experienced CHRONIC INFLAMMATORY DEMYELINATING POLYRADICULONEUROPATHY (developed exacerbated CIDP (or AIDP)) (seriousness criterion medically significant) and GUILLAIN-BARRE SYNDROME (developed exacerbated CIDP (or AIDP)) (seriousness criterion medically significant). At the time of the report, CHRONIC INFLAMMATORY DEMYELINATING POLYRADICULONEUROPATHY (developed exacerbated CIDP (or AIDP)) had resolved and GUILLAIN-BARRE SYNDROME (developed exacerbated CIDP (or AIDP)) outcome was unknown. Not Provided    DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, CSF protein: abnormal (abnormal) Elevated CSF protein.. On an unknown date, Electromyogram: abnormal (abnormal) Electromyography revealed F-waves in the lower extremities.. On an unknown date, Electrophoresis protein: normal (normal) Unremarkable. On an unknown date, Glycosylated haemoglobin: normal (normal) Unremarkable. On an unknown date, Physical examination: abnormal (abnormal) notable for normal strength in the upper extremities, with diminished but present reflexes. Lower extremity exam revealed bilateral weakness in the hip, distal extremities, and feet. Ankle and knee reflexes were absent.. On an unknown date, Syphilis: normal (normal) Unremarkable. On an unknown date, Thyroid function test: normal (normal) Unremarkable.     The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown) was unknown.   For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter considered CHRONIC INFLAMMATORY DEMYELINATING POLYRADICULONEUROPATHY (developed exacerbated CIDP (or AIDP)) and GUILLAIN-BARRE SYNDROME (developed exacerbated CIDP (or AIDP)) to be possibly related.    In 2017, shortly after receiving influenza vaccine, he had symptoms, laboratory, and electrophysiological features of GBS/AIDP. Thyroid function tests, hemoglobin A1c, serum protein electrophoresis, syphilis and Lyme titers were unremarkable. He had incomplete resolution of symptoms with a course of five plasma exchange treatments. One month after initial diagnosis and treatment, it was determined that his condition had evolved to CIDP, for which he received ongoing plasmapheresis on an every two week basis, with symptom improvement but not normalization.  This case concerns a 87-year-old male patient with a relevant medical history of Guillain-Barre syndrome and Chronic inflammatory demyelinating polyradiculoneuropathy, who experienced the unexpected serious event/AESI of Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and Guillain-Barre syndrome (GBS/AIDP) and after mRNA- 1273 Moderna vaccine (Moderna COVID-19 Vaccine / Spikevax). The event occurred approximately 21 days after the dose of mRNA- 1273 vaccine, dose number unknown. Clinical manifestations were weakness and hypoesthesia in lower limbs. Ankle and knee reflexes were absent. Cerebrospinal fluid showed elevated protein. Electromyography revealed F-waves in the lower extremities. Acute on chronic  inflammatory demyelinating polyradiculoneuropathy was diagnosed (GBS/AIDP or exacerbation of CIDP). Treatment with plasmapheresis was intensified with resolution of the event. The rechallenge is not applicable since only information about the one dose was disclosed. Underlying Chronic inflammatory demyelinating polyradiculoneuropathy and previous Guillain-Barre syndrome remain a confounder. The benefit-risk relationship of mRNA- 1273 vaccine / Moderna COVID-19 Vaccine is not affected by this report.   Reporter did not allow further contact; Sender's Comments: This case concerns a 87-year-old male patient with a relevant medical history of Guillain-Barre syndrome and Chronic inflammatory demyelinating polyradiculoneuropathy, who experienced the unexpected serious event/AESI of Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and Guillain-Barre syndrome (GBS/AIDP) and after mRNA- 1273 Moderna vaccine (Moderna COVID-19 Vaccine / Spikevax). The event occurred approximately 21 days after the dose of mRNA- 1273 vaccine, dose number unknown. Clinical manifestations were weakness and hypoesthesia in lower limbs. Ankle and knee reflexes were absent. Cerebrospinal fluid showed elevated protein. Electromyography revealed F-waves in the lower extremities. Acute on chronic  inflammatory demyelinating polyradiculoneuropathy was diagnosed (GBS/AIDP or exacerbation of CIDP). Treatment with plasmapheresis was intensified with resolution of the event. The rechallenge is not applicable since only information about the one dose was disclosed. Underlying Chronic inflammatory demyelinating polyradiculoneuropathy and previous Guillain-Barre syndrome remain a confounder. The benefit-risk relationship of mRNA- 1273 vaccine / Moderna COVID-19 Vaccine is not affected by this report.