Abstract
The hospitalization rate for pneumonia among adults has decreased in the U.S. since the early 2000's. The increasing prevalence of chronic conditions among adults, especially those associated with risk factors for pneumonia caused by Streptococcus pneumoniae, influenza and respiratory syncytial virus, may be contributing to a larger population susceptible to pneumonia. Existing estimates utilizing claims data may underestimate the true rates of disease due to how diagnostic coding fields are used to define outcomes. A retrospective descriptive study of adult (> 18 years) ACPI inpatient hospitalizations was conducted using US healthcare claims data from 2014 to 2023. Principal ACPI hospitalizations were defined as those with a principal field code for ACPI only. Expanded principal ACPI hospitalizations were those with a principal code for ACPI or a principal code for sepsis, meningitis, COPD, empyema, and acute respiratory failure in combination with a secondary code for ACPI. Prevalence of risk factors indicative for pneumococcal pneumonia, RSV, or severe influenza vaccination were assessed. Principal ACPI showed the largest decline in incidence and this decline was greatest among the younger age groups compared to the older adults. In contrast, the expanded principal definition of ACPI captured between 48.5 and 63.4% of ACPI codes present in any field and showed lower variation over time compared to the principal definition. The proportion of adults with a risk factor of interest increased during the study period, particularly between 2020 and 2023. Greater than 96% of adults ≥ 65y hospitalized with ACPI in 2023 had at least one risk factor indicative for vaccination against pneumococcal, influenza, or RSV in the 3 years prior. We show that using only principal diagnostic codes to identify ACPI hospitalization may disproportionately inflate the decline in ACPI hospitalizations between 2014 and 2023. Using a principal ACPI hospitalization definition would exclude approximately 30% of hospitalizations that would meet the expanded principal ACPI definition. The shift towards greater use of sepsis and COPD codes in the principal diagnostic code position may be attributed to coding changes over the study period. Risk factors for pneumonia increased in prevalence, particularly during the COVID-19 pandemic period.