(333) Background rates of adverse events of special interest for COVID-19 vaccine safety monitoring in South Korea

Background: Background rates are crucial for identifying potential increased risks of adverse events of special interest (AESI) following vaccination.

Objectives: To estimate background incidence rates (IR) for AESI associated with COVID-19 vaccines in South Korea between 2015 and 2019.

Methods: We used the National Health Insurance Service-National Sample Cohort (NHIS-NSC), representing approximately one million Korean citizens. The study spanned from January 1, 2015, to December 31, 2019, with IR calculated annually. We defined a list of AESIs potentially related to COVID-19 vaccines and evaluated 8 AESIs, including acute myocarditis, acute pericarditis, acute disseminated encephalomyelitis (ADEM), acute transverse myelitis (ATM), encephalitis, anaphylaxis, Bell’s palsy, and lymphadenitis. The index date for each year was defined as January 1. We established a clean window before each index date, during which no AESI was observed, set for 365 days for all events, except for anaphylaxis (30 days), and encephalitis and Bell’s palsy (183 days). Two operational strategies were implemented: the first employs a broad definition, utilizing ICD-10 codes for either primary or secondary diagnoses to enhance sensitivity; the second adopts a narrow definition, integrating ICD-10 codes with procedure and drug codes to increase accuracy and specificity. Individuals were followed from the index date until the occurrence of a health event, death, or the end of the calendar year, whichever came first. For conditions with a clean window of less than 365 days, individuals could re-enter the cohort within the same year and were treated as separate observations. The IR, calculated by dividing the number of incident cases by the total person-time at risk, was expressed per 100,000 person-years (PY), with separate calculations conducted for sex and age group.

Results: During the study period, the incidence of anaphylaxis, Bell's palsy, and lymphadenitis under the broad definition continued to increase, ranging from 40.91 to 59.17, 113.07 to 137.79, and 1949.82 to 2236.93 per 100,000 PY, respectively. 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.