Background Rates of Adverse Events of Special Interest for COVID-19 Vaccine Safety Monitoring in South Korea

Abstract

Objectives: This study aimed to estimate background incidence rates (IR) for adverse events of special interest (AESI) associated with COVID-19 vaccines in South Korea. Methods: We used data from the Korean National Health Insurance Service-National Sample Cohort from January 1, 2015 to December 31, 2019, with IR calculated annually. Eight AESIs including acute myocarditis, acute pericarditis, acute disseminated encephalomyelitis (ADEM), acute transverse myelitis (ATM), encephalitis, anaphylaxis, Bell’s palsy, and lymphadenitis were evaluated. Two operational strategies were implemented: a broad definition using ICD-10 codes for primary or secondary diagnoses to enhance sensitivity, and a narrow definition integrating ICD-10 codes with procedure and drug codes for enhanced accuracy and specificity. IR was calculated by dividing the number of incident cases by the total person-time at risk, with separate analyses by sex and age. Results: During the study period, the incidence of anaphylaxis, Bell’s palsy, and lymphadenitis under the broad definition continued to increase. Under the narrow definition, there were no reported cases of acute myocarditis or Bell’s palsy from 2015 to 2019. Additionally, the incidence rates for acute pericarditis, ADEM, ATM, encephalitis, anaphylaxis, and lymphadenitis were lower than those calculated under the broad definition. Males generally had higher rates, except for Bell’s palsy and lymphadenitis. Most AESIs were more common in individuals aged 65 and older, whereas ADEM and lymphadenitis were higher in children aged 0-19 years. Conclusions: Variability in AESI rates emphasizes the importance of considering definitions and demographics when establishing background rates for surveillance, which is a critical aspect of monitoring vaccine safety.