Characterizing persistent Post-COVID-19 vaccination symptoms using MedDRA system organ class and preferred term classifications.

Post-COVID-19 Vaccination Syndrome (PCVS) refers to persistent, multisystemic symptoms developing after SARS-CoV-2 immunization. We established a nationwide registry across 14 Japanese outpatient clinics and analyzed 179 “clinically definitive” cases from 279 enrollees (December 2020–August 2023). All adverse events (AEs) were coded using MedDRA/J Preferred Terms and System Organ Classes; severity followed NCI-CTCAE V5.0. The cohort (66.5% female; mean age 59 years) experienced 493 AEs (median 2 per patient, range 1–29). Three System Organ Classes—General Disorders (29.2%), Nervous System Disorders (22.3%), and Musculoskeletal Disorders (10.1%)—accounted for 61.7% of events; fatigue, brain fog, dizziness, and extremity pain predominated. While 69.4% of AEs arose within 90 days post-vaccination, 12.4% appeared ≥ 360 days later. Severe AEs (≥ Grade 3) occurred in 14.6% of patients; overall improvement was 65.1%, leaving 29.4% unresolved. A provisional phenotype classification combining symptom patterns, onset timing, and severity identified high-risk subgroups with > 60% non-recovery rates. These findings underscore the need for enhanced post-vaccination surveillance systems and comprehensive care frameworks specifically tailored to address the protean manifestations and persistent nature of PCVS.