(A-252) Incidence of Outcomes Relevant to Vaccine Safety Monitoring in a General Population, After Influenza Vaccination, and After Respiratory Infections in US Claims Data from August 1, 2022 to July 31, 2023

Background: Background rates of safety outcomes can be used to contextualize the benefits and risks associated with vaccination. As new vaccines are developed to prevent respiratory virus infections, contemporary background rates for outcomes assessed in vaccine safety surveillance during a COVID-19 endemic period are needed.

Objectives: To estimate incidence rates of outcomes relevant to vaccine safety monitoring in a general population, after influenza vaccination, and after respiratory infections.

Methods: This retrospective cohort study used three US claims data sources to estimate background rates of 30 safety outcomes in five cohorts between 1 August 2022 and 31 July 2023. The cohorts included 1) a general population (all ages) with an index date of 1st day of enrollment, 2) recipients with an influenza vaccine (index date), and 3) those with medically attended infection of a) SARS-CoV-2, b) respiratory syncytial virus (RSV), and c) influenza, with the index date as the date of infection. Individuals with missing age or sex, and those without continuous medical and pharmacy enrollment in the year prior to the index date were excluded. Predefined outcome washout was applied to each cohort. Baseline demographics, clinical characteristics, and healthcare utilization were reported. Incidence rates (IRs) and 95% confidence intervals were estimated overall and stratified by age and sex.

Results: Generally, individuals with a medically attended infection had a higher prevalence of baseline clinical comorbidities compared to the general or influenza-vaccinated cohorts. Across outcomes overall, estimated IRs were similar or lower among the influenza-vaccinated cohort, compared to the general population, while IRs were higher among all medically attended infection cohorts. Among adults ≥65 years, heart failure, atrial fibrillation, and ischemic coronary artery disease had the highest observed IRs across all cohorts. Among adults 18-64 years, seizures, myocarditis, and anaphylaxis had the greatest IRs following respiratory infections. Among children <18 years, outcomes were rarely observed, though seizure was the highest estimated IR across cohorts. IRs were typically similar or slightly higher in males than females across ages.

Conclusions: This study estimated IRs of outcomes relevant for vaccine safety monitoring in a COVID-19 endemic period across multiple populations and data sources, informing future vaccine safety surveillance and aiding relative benefit-risk assessment. Within cohorts, observed rates differed across age and sex groups, highlighting the need to consider demographic characteristics of the target population when applying estimated background rates
for pharmacovigilance.