Abstract
Background
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.
Methods
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.
Results
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.
Conclusions
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.
Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1186/s41479-026-00207-0.
Keywords: All-cause pneumonia, Influenza, Claims, Diagnostic codes, Comorbidities
Introduction
In 2011, all-cause pneumonia was the most frequent cause of non-childbirth-related hospitalization in the US, comprising 2.87% of all non-childbirth hospitalizations. By 2018, pneumonia had declined to the fifth leading cause, making up 2.08% of hospitalizations [1]. This decline has been attributed, in part, to the greater number and improved efficacy of respiratory vaccinations available for children and adults [2]. In the US, influenza, respiratory syncytial virus (RSV), and pneumococcal vaccines are recommended for adults to prevent respiratory illnesses that can cause pneumonia and related severe diseases. Pneumococcal vaccines have been recommended to high risk adults since 1984 and influenza vaccines have been universally recommended for adults in the US since 2010 [3–5]. In 2023, the Advisory Committee on Immunization Practices (ACIP) stated that adults over 60 years of age may receive a single dose of the RSV vaccine through shared clinical decision making, with an update in 2025 expanding this recommendation to include adults 50–59 years of age at increased risk for severe RSV disease [6, 7]. Despite the increase in available respiratory vaccines to prevent respiratory infections and pre-COVID-19 pandemic declines in pneumonia hospitalizations, low vaccine uptake among adults in the US necessitates frequent assessments of trends in pneumonia hospitalization [8–10].
Many risk factors for pneumonia are also indications for adult respiratory vaccination. For example, chronic lung disease, chronic kidney disease, and diabetes, are all comorbid conditions that may place an adult at greater risk for pneumococcal pneumonia, influenza, and RSV [11–13]. The prevalence of some risk factors, like chronic obstructive pulmonary disease (COPD), have remained stable in the U.S. but others, like diabetes have increased into the late 2010s and are expected to continue increasing into 2030 [14–17]. Because multimorbidity increases with increasing age, and the size of the population 65 years and older continues to grow, the absolute number of U.S. adults at risk for pneumonia are anticipated to continue to increase with time [18–20].
Crucially, the reported decline in pneumonia hospitalizations is not equal across all age groups, nor is the definition of pneumonia hospitalization consistent across studies. One driver of the potential discrepancy in our understanding of trends is the method by which pneumonia is classified when analyzing medical claims data. Studies often examine International Classification of Diseases (ICD) codes listed in the principal diagnostic code field, as this field is intended to capture the principal diagnosis responsible for admission. Strictly defining pneumonia by the principal code means only a portion of the total number of medical claims for inpatient pneumonia are captured [21]. Furthermore, the COVID-19 pandemic altered respiratory disease transmission cycles and social contact trends, thus affecting rates of non-COVID pneumonia hospitalizations [22]. This study aims to characterize trends in all-cause pneumonia and influenza (ACPI) hospitalization rates by diagnostic coding field position and trends in specific comorbidities among ACPI hospitalized patients pre- and post-COVID-19 pandemic. These results can aid in our understanding and predictions of future ACPI trends as new respiratory vaccines enter the market.
Methods
Study design and data
This study was a retrospective analysis of ACPI inpatient hospitalizations identified through US healthcare claims data between January 1, 2014, and December 31, 2023. The data source was Merative™ MarketScan® Commercial Database with Medicare Supplement A (hereafter “MarketScan”). Included in the data are deidentified inpatient and outpatient claims for healthcare services, basic patient demographics, as well as claims for long-term care services and prescription drugs. The full MarketScan dataset includes patient-level information on approximately 293 million US individuals that are insured commercially, while Medicare Supplement A includes healthcare interactions for the subset of retirees that have Medicare supplemental insurance paid by employers [23, 24]. Enrollees with Medicare supplemental insurance constituted approximately 14% of all Medicare receiving retirees in 2010 [25]. Full-year data was available for the years between January 1, 2011, and December 31, 2023.
The population of interest was restricted to adults (> 18y) with at least two complete years of enrollment in MarketScan data (i.e., fully covered for more than 25 days per month and enrolled in all months of a year for at least two consecutive years). This restriction was performed to stabilize the underlying population, as the number of unique enrollees decreased each year of the study period and consistent enrollment during the lookback period was necessary to assess comorbidity history [26] (Supplementary Figure S1).
Outcomes
Enrollees were then restricted to those with a hospitalization for all-cause pneumonia or influenza (ACPI), identified by presence of an International Classification of Diseases code, ICD-9-CM or ICD-10-CM, for ACPI in any diagnostic coding field (Supplementary Table S1). Hospitalizations with ICD codes for COVID-19 were excluded from analysis to allow for comparison of ACPI pre- and post-2020. Influenza was included as part of the ACPI definition to broaden our understanding of hospitalized respiratory disease trends and address the close relationship between influenza and the development of pneumonia. Because the study period covered the transition from ICD-9 to ICD-10 in October 2015, crosswalks for diagnostic codes were made according to forward and reverse general equivalence mapping using the Agency for Healthcare Research and Quality’s (AHRQ) Map It software [27]. ACPI hospitalizations (with any ACPI code of interest in any diagnostic code position) that occurred within 90 days of a previous ACPI hospitalization were excluded to prevent inclusion of relapsing ACPI outcomes.
ACPI outcomes were defined by the position in which the diagnostic code was found in the inpatient summary medical claims data. Summary inpatient claims data have one principal diagnostic coding field and 14 secondary diagnostic coding fields while outpatient claims data have one principal field and four secondary fields. Diagnostic codes were classified as any position ACPI, principal ACPI (denoted 1˚), expanded principal ACPI (denoted 1+), and secondary only ACPI (denoted 2˚). Any position ACPI was the most inclusive categorization, indicating that an ICD code for ACPI was found in any of the 15 inpatient diagnostic coding fields. Principal ACPI (1˚) includes patients where a diagnostic code for ACPI was in the principal diagnostic coding field (DX1), regardless of codes present in secondary fields. Expanded principal ACPI (1+) encompasses individuals with a principal diagnostic code for sepsis, acute respiratory failure, meningitis, pleural empyema, or COPD, in combination with a secondary diagnostic code for ACPI or hospitalizations with a principial diagnostic code for ACPI (1˚). Severe cases of pneumonia may be more likely to have a principal code for meningitis, respiratory failure, and septicemia [28]. The 1 + classification also addresses changing standards by the American Hospital Association (AHA) for COPD coding when a lower respiratory infection is also present and recent research indicating that the switch from ICD-9 to ICD-10 resulted in shifts in pneumonia coding practices that allowed for higher reimbursement for respiratory failure [21, 29–31]. Secondary only ACPI (2˚) groups all patients with a code for ACPI that is present only in secondary diagnostic fields, making this definition mutually exclusive with principal definition (1˚) but overlapping with the expanded principal ACPI definition.
Risk factors for pneumonia were identified through both inpatient and outpatient healthcare claim codes present in any diagnostic coding field position during a 3-year lookback period prior to incident ACPI hospitalization. Comorbidities investigated included known risk factors for pneumonia that overlapped with conditions specified in vaccine recommendations (as defined by the ACIP recommendations and CDC) [11, 32]). Overlapping risk factors for pneumococcal infection, RSV infection and severe influenza infection include chronic heart disease, chronic liver disease, chronic lung disease, chronic kidney disease, diabetes mellitus, immunosuppression (including long term steroid use and solid organ transplants), cancer (including solid organ malignancy, leukemia, and lymphoma), asplenia, HIV/AIDS, and hemoglobinopathies (e.g., sickle cell disease). The full list of conditions by vaccine indication is found in (Supplementary Table S2).
Comorbidities listed concurrently with the index ACPI hospitalization did not contribute to the comorbidity prevalence because it was not discernable if the code represented a new or acute disease state related to the ACPI hospitalization. If a patient was hospitalized for ACPI and was a MarketScan data enrollee but had no previous inpatient or outpatient records in the three years prior to ACPI, they were recorded as having no risk factors of interest. The total list of comorbidities assessed, and their respective ICD-9 and ICD-10 codes are shown in Supplementary Table S3.
Descriptive statistics were provided by calendar year and age group (19-34y, 35-49y, 50-64y, 65-74y, ≥ 75y) for each objective. The primary objective of this study was to quantify the change in ACPI hospitalization by ACPI definition and changes to underlying comorbidity prevalence from 2014 to 2023. To accommodate for the 3-year lookback period for comorbidities, the ACPI outcome period began on January 1, 2014, and extended to December 31, 2023. Comorbidity trends were then analyzed individually and plotted by calendar year and age group. Annual incidence rates and relative change in incidence rates were also calculated.
Results
In total, 645,338 hospitalizations with ACPI present in any diagnostic field occurred among 549,018 unique individuals enrolled in MarketScan data and who met our continuous enrollment criteria between 2014 and 2023. The overall incidence rate (IR) showed a decreasing trend over time, from 5.6 ACPI hospitalizations per 1,000 enrollees in 2014, to 3.9 hospitalizations per 1,000 enrollees in 2023 (Table 1). When stratified by age, all groupings assessed showed an overall decline in incidence between 2014 and 2023 (with fluctuations in 2020 among the ≥ 65y age groupings) and increasing incidence with increasing age (Fig. 1).
Table 1.
Characteristics of ACPI hospitalizations (any position)
| Variable | Year | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | Total | |
| Enrollees | 23,491,336 | 16,189,686 | 15,844,401 | 14,668,190 | 14,120,171 | 13,172,992 | 13,300,708 | 12,371,391 | 11,917,620 | 10,013,487 | 145,089,982 |
|
ACPI Hospitalizations N |
130,538 | 90,978 | 83,916 | 69,730 | 55,535 | 50,322 | 51,590 | 36,962 | 36,964 | 38,803 | 645,338 |
|
Age Mean (Median) |
67 (68) | 68 (70) | 67 (68) | 66 (66) | 62 (62) | 62 (61) | 65 (67) | 66 (67) | 67 (69) | 68 (71) | 66 (67) |
|
Age Group N |
|||||||||||
| 19-34y | 6111 | 4325 | 4472 | 4074 | 4096 | 4130 | 3093 | 2141 | 1799 | 1497 | 35,738 |
| 35-49y | 12,979 | 8235 | 8413 | 7325 | 7327 | 7020 | 5633 | 3745 | 3516 | 3493 | 67,686 |
| 50-64y | 39,673 | 25,926 | 24,894 | 22,257 | 21,310 | 19,371 | 15,737 | 11,266 | 11,049 | 10,841 | 203,324 |
| 65-74y | 21,718 | 14,489 | 13,727 | 10,573 | 6637 | 5688 | 8570 | 6254 | 6010 | 6341 | 100,007 |
| ≥ 75y | 50,057 | 38,003 | 32,410 | 25,501 | 16,165 | 14,113 | 18,557 | 13,556 | 14,590 | 16,631 | 239,583 |
| % Male | 49% | 49% | 49% | 49% | 49% | 49% | 51% | 52% | 50% | 50% | 49% |
| Median Length of enrollment (years) | 3 | 4 | 5 | 6 | 6 | 6 | 7 | 9 | 10 | 10 | 5 |
Fig. 1.
Annual Incidence of ACPI per 1,000 enrollees by definition and age category. Any ACP is defined as a diagnostic code for pneumonia or influenza in any diagnostic field position. Principal ACPI (1˚)had a code for ACPI in the principal diagnostic coding position. Expanded principal definition of ACPI (1+) is defined as a principal diagnostic code for ACPI or a principal diagnostic code for sepsis, meningitis, chronic obstructive pulmonary disease (COPD), empyema, or respiratory failure in combination with a secondary diagnostic code for pneumonia. The secondary code, (2˚) definition shows only hospitalizations where the ACPI code was found only in a secondary DX position (DX2-DX15)
Principal ACPI (1˚) constituted 28.7–34.7% of all ACPI diagnostic codes in 2014 and 17.4% − 28.3% of all ACPI in 2023 (Table 2). 1˚ ACPI showed the largest decline in incidence among all definitions evaluated, with the largest reductions observed in the younger age groups compared to the older adults. Adults in the 19-34y age cohort and the 35-49y age cohort had a 65.6% and 60.9% decrease, respectively, in incidence between 2014 and 2023, compared to only a 31.9% decrease among adults aged 65-74y (Table 3). Conversely, secondary only ACPI codes (2˚) increased with time.
Table 2.
Annual incidence rate of all-cause pneumonia by diagnostic code position, 2014 to 2023
| Diagnostic Code Position by Age Group | Incidence Rate per 1,000 enrollees | % Relative change in incidence | |||||
|---|---|---|---|---|---|---|---|
| Average (all years) | 2014 | 2019 | 2023 | 2014–2019 | 2014–2023 | 2019–2023 | |
| Principal (1˚) | 1.24 | 1.75 | 1.04 | 1.00 | -40.57 | -42.86 | -3.85 |
| Secondary (2˚) | 3.21 | 3.81 | 2.78 | 2.88 | -27.03 | -24.41 | -3.60 |
| Expanded Principal definition (1+) | 2.63 | 3.20 | 2.31 | 2.33 | -27.81 | -27.19 | -0.87 |
| ACPI in any field | 4.45 | 5.56 | 3.82 | 3.88 | -31.29 | -30.22 | -1.57 |
| 19-34y | |||||||
| Principal (1˚) | 0.23 | 0.32 | 0.26 | 0.11 | -18.75 | -65.63 | -57.69 |
| Secondary (2˚) | 0.72 | 0.79 | 0.87 | 0.52 | + 10.13 | -34.18 | -40.23 |
| Expanded Principal definition (1+) | 0.47 | 0.54 | 0.60 | 0.31 | + 11.11 | -42.59 | -48.33 |
| ACPI in any field | 0.95 | 1.11 | 1.13 | 0.63 | + 1.80 | -43.24 | -44.25 |
| 35-49y | |||||||
| Principal (1˚) | 0.44 | 0.64 | 0.46 | 0.25 | -28.13 | -60.94 | -45.65 |
| Secondary (2˚) | 1.11 | 1.21 | 1.24 | 0.89 | + 2.48 | -26.45 | -28.23 |
| Expanded Principal definition (1+) | 0.88 | 1.05 | 0.98 | 0.65 | -6.67 | -38.10 | -33.67 |
| ACPI in any field | 1.55 | 1.85 | 1.71 | 1.15 | -7.57 | -37.84 | -32.75 |
| 50-64y | |||||||
| Principal (1˚) | 1.12 | 1.51 | 1.15 | 0.76 | -23.84 | -49.67 | -33.91 |
| Secondary (2˚) | 2.97 | 3.34 | 3.12 | 2.42 | -6.59 | -27.54 | -22.44 |
| Expanded Principal definition (1+) | 2.43 | 2.80 | 2.59 | 1.89 | -7.50 | -32.86 | -27.03 |
| ACPI in any field | 4.09 | 4.86 | 4.27 | 3.19 | -12.14 | -34.36 | -25.29 |
| 65-74y | |||||||
| Principal (1˚) | 3.36 | 4.08 | 3.22 | 2.78 | -21.08 | -31.86 | -10.87 |
| Secondary (2˚) | 9.11 | 9.51 | 8.66 | 7.96 | -8.94 | -16.30 | -8.08 |
| Expanded Principal definition (1+) | 7.74 | 8.22 | 7.52 | 6.80 | -8.52 | -17.27 | -9.57 |
| ACPI in any field | 12.47 | 13.59 | 11.88 | 10.73 | -12.58 | -21.04 | -9.68 |
| ≥ 75y | |||||||
| Principal (1˚) | 10.66 | 13.52 | 10.95 | 7.92 | -19.01 | -41.42 | -27.67 |
| Secondary (2˚) | 26.08 | 29.07 | 27.33 | 20.09 | -5.99 | -30.89 | -26.49 |
| Expanded Principal definition (1+) | 22.06 | 24.58 | 23.87 | 17.14 | -2.89 | -30.27 | -28.19 |
| ACPI in any field | 36.73 | 42.59 | 38.28 | 28.01 | -10.12 | -34.23 | -26.83 |
Note: Incidence ACPI hospitalizations per 1,000 enrollees. Expanded primary definition of ACP is defined as a primary diagnostic code for pneumonia/influenza or a primary diagnostic code for sepsis, meningitis, chronic obstructive pulmonary disease (COPD), empyema, or acute respiratory failure and a secondary diagnostic code for pneumonia
Table 3.
Relative change in proportion of ACPI by diagnostic code position, 2014 to 2023
| Diagnostic Code Position by Age | Proportion of total ACP by coding position definition (%) | Relative % Change | |||||
|---|---|---|---|---|---|---|---|
| Average | 2014 | 2019 | 2023 | 2014–2019 | 2014–2023 | 2019–2023 | |
| 19-34y | |||||||
| Principal (1˚) | 24.13% | 28.67% | 22.86% | 17.43% | -20.27 | -39.19 | -23.76 |
| Secondary (2˚) | 75.87% | 71.33% | 77.14% | 82.57% | 8.15 | 15.75 | 7.04 |
| Expanded Principal definition (1+) | 49.70% | 48.83% | 53.44% | 48.50% | 9.44 | -0.68 | -9.24 |
| 35-49y | |||||||
| Principal (1˚) | 28.65% | 34.76% | 27.05% | 22.04% | -22.19 | -36.59 | -18.52 |
| Secondary (2˚) | 71.35% | 65.24% | 72.95% | 77.96% | 11.82 | 19.50 | 6.87 |
| Expanded Principal definition (1+) | 56.40% | 56.55% | 57.64% | 56.28% | 1.93 | -0.46 | -2.36 |
| 50-64y | |||||||
| Principal (1˚) | 27.45% | 31.13% | 26.98% | 23.95% | -13.35 | -23.09 | -11.23 |
| Secondary (2˚) | 72.55% | 68.87% | 73.02% | 76.05% | 6.04 | 10.44 | 4.15 |
| Expanded Principal definition (1+) | 59.25% | 57.68% | 60.62% | 59.05% | 5.09 | 2.38 | -2.59 |
| 65-74y | |||||||
| Principal (1˚) | 26.95% | 30.04% | 27.13% | 25.88% | -9.69 | -13.85 | -4.61 |
| Secondary (2˚) | 73.05% | 69.96% | 72.87% | 74.12% | 4.16 | 5.95 | 1.72 |
| Expanded Principal definition (1+) | 62.09% | 60.47% | 63.31% | 63.35% | 4.69 | 4.76 | 0.06 |
| ≥ 75y | |||||||
| Principal (1˚) | 29.02% | 31.74% | 28.60% | 28.27% | -9.89 | -10.94 | -1.12 |
| Secondary (2˚) | 70.98% | 68.26% | 71.40% | 71.73% | 4.60 | 5.09 | 0.46 |
| Expanded Principal definition (1+) | 60.04% | 57.72% | 62.37% | 61.19% | 8.05 | 6.01 | -1.89 |
Note: Proportions are out of total hospitalizations for ACPI recorded in any diagnostic coding position. Expanded primary definition of ACP is defined as a primary diagnostic code for pneumonia/influenza or a primary diagnostic code for sepsis, meningitis, chronic obstructive pulmonary disease (COPD), empyema, or acute respiratory failure and a secondary diagnostic code for pneumonia
The expanded principal definition (1+) of ACPI captured between 48.5% and 63.4% of ACPI defined in any position throughout the study period and showed less variation over time compared to the 1˚ definition. In 2023, the incidence of ACPI using the 1 + definition was more than double the 1˚ incidence rate across all age groups. Between 2014 and 2019, the 19-34y age group showed an 11.1% increase in 1 + ACPI incidence while older age groups showed smaller decreases in incidence. Furthermore, the divergence between declining incidence in the 1˚ definition compared the 1 + definition was most apparent among younger adults. Between 2014 and 2023, the relative change in proportion of all pneumonia cases that met the 1 + definition showed very little, -0.46 to -0.68% in the younger age groups, and a maximum of + 6% change in the older adults (Tables 2 and 3).
Prior to 2017, the most common diagnostic code in the first DX field for an any position ACPI hospitalization was unspecified pneumonia and, beginning in 2017, the most common diagnostic code was for unspecified sepsis, followed by unspecified pneumonia. This ordering held true through the COVID-19 pandemic (COVID-19 hospitalizations were excluded from this analysis).
The most prevalent risk factors assessed by age group were chronic heart disease (≥ 75y), and chronic lung disease or asthma (65-74y, 50-64y, 35-49y, 19-34y) (Supplementary Table S5, Fig. 2). Some comorbidities increased in prevalence, like history of obesity, chronic liver disease, and malignant tumors, while others, like chronic heart disease, and cigarette use declined in most age groups. Diabetes prevalence among adults 50-74y and CKD prevalence among adults 35-64y did not vary greatly over the course of the study period (< 4% change). Approximately 6% of 1 + ACPI hospitalizations among adults 50-64y and 15% of adults 35-49y did not have a risk factor for pneumonia in 2023, a decrease in the risk-free population of approximately 4 and 8% points since 2014, respectively (Table 4). 78.5% of adults aged 19-34y with pneumonia hospitalized in 2023 had a risk factor indicative for receiving the pneumococcal, influenza, or RSV vaccine. Almost all adults ≥ 65y hospitalized with ACPI in 2023 had at least one risk factor that was indicative for pneumococcal, influenza, or RSV vaccine recorded in the 3 years prior.
Fig. 2.
Trends in comorbidities among ACPI patients with an expanded primary hospitalization (1 + APCI). Expanded principal, 1+, ACPI is defined as a primary diagnostic code for pneumonia or influenza or a principal diagnostic code for sepsis, meningitis, chronic obstructive pulmonary disease (COPD), or respiratory failure in combination with a secondary diagnostic code for pneumonia
Table 4.
Percentage of ACP hospitalizations (expanded primary definition) with comorbidities grouped by indication for vaccine
| Vaccine-specific Risk Group | Year | Relative % Change | ||||
|---|---|---|---|---|---|---|
| 2014 | 2019 | 2023 | 2014–2019 | 2014–2023 | 2019–2023 | |
| 19-34y | ||||||
| Pneumococcal Vaccine Risk Factors | 63.27% | 62.62% | 67.72% | -1.03 | 6.24 | 7.34 |
| RSV Vaccine Risk Factors | 69.47% | 71.86% | 76.17% | 3.44 | 9.64 | 6.00 |
| Influenza Vaccine Risk Factors | 70.64% | 72.59% | 77.41% | 2.75 | 9.58 | 6.64 |
| Any of above | 71.21% | 73.22% | 77.82% | 2.82 | 9.28 | 6.29 |
| 35-49y | ||||||
| Pneumococcal Vaccine Risk Factors | 72.22% | 75.21% | 78.54% | 4.14 | 8.75 | 4.42 |
| RSV Vaccine Risk Factors | 76.54% | 81.02% | 84.54% | 5.86 | 10.45 | 4.34 |
| Influenza Vaccine Risk Factors | 77.22% | 81.46% | 85.15% | 5.50 | 10.27 | 4.52 |
| Any of above | 77.44% | 81.81% | 85.55% | 5.65 | 10.48 | 4.58 |
| 50-64y | ||||||
| Pneumococcal Vaccine Risk Factors | 87.38% | 88.50% | 90.46% | 1.29 | 3.52 | 2.21 |
| RSV Vaccine Risk Factors | 89.56% | 91.59% | 93.69% | 2.28 | 4.62 | 2.29 |
| Influenza Vaccine Risk Factors | 90.10% | 92.02% | 93.94% | 2.13 | 4.26 | 2.08 |
| Any of above | 90.27% | 92.22% | 94.14% | 2.16 | 4.30 | 2.09 |
| 65-74y | ||||||
| Pneumococcal Vaccine Risk Factors | 94.74% | 94.72% | 96.22% | -0.02 | 1.56 | 1.58 |
| RSV Vaccine Risk Factors | 95.84% | 96.33% | 97.34% | 0.52 | 1.57 | 1.04 |
| Influenza Vaccine Risk Factors | 96.38% | 96.89% | 97.76% | 0.53 | 1.44 | 0.90 |
| Any of above | 96.41% | 96.97% | 97.81% | 0.58 | 1.45 | 0.86 |
| ≥ 75y | ||||||
| Pneumococcal Vaccine Risk Factors | 94.17% | 95.19% | 96.08% | 1.09 | 2.03 | 0.93 |
| RSV Vaccine Risk Factors | 95.24% | 96.28% | 97.24% | 1.09 | 2.09 | 0.99 |
| Influenza Vaccine Risk Factors | 97.16% | 98.05% | 98.46% | 0.91 | 1.33 | 0.42 |
| Any of above | 97.18% | 98.08% | 98.48% | 0.93 | 1.34 | 0.40 |
Note: Percentage among the expanded definition, 1+, of ACPI hospitalization
Discussion
In this descriptive study of ACPI hospitalizations, we demonstrate that the choice of diagnostic coding field criteria can influence the magnitude and direction of observed trends. The expanded principal definition of ACPI hospitalization was the more stable definition, in terms of capturing a steady proportion of total APCI codes over time, while also showing the general decline in pneumonia hospitalization between 2014 and 2023. For the 19 to 34y age group, the choice to use a principal only code to identify ACPI between 2014 and 2019 would result in a perceived decline in incidence, while the incorporation of secondary codes revealed minor increases in ACPI.
Multiple shifts to coding practices occurred throughout the time period of the study which provided a unique time period to assess trends in ACPI. The first change to coding occurred in late 2015 with the switch from ICD-9 to ICD-10 in the US. Other studies have shown that the ICD-9/ICD-10 coding shift in 2015 would require an adjustment factor of approximately 7% when estimating pneumonia in adults [31]. Additionally, our data showed that the diagnostic code for COPD with LRTI (J44.0: COPD with acute LRTI) moved from the fifth most common principal diagnostic code in 2016 to the third most common in 2017. This reflects the October 2016 coding change by the AHA that required COPD to be coded in the primary position in the event of a lower respiratory tract infection with COPD although there is some room for flexibility given clinical notes for pneumonia. Previously, these hospitalization events could be coded with LRTI in the primary position and thus be captured in a principal only definition of ACPI [30, 33]. Our study also found that principal diagnoses like septicemia surpassed codes for unspecified pneumonia to be the most frequent code for ACPI hospitalization in 2017. Other studies have shown an increase in septicemia coding with one showing that between 2009 and 2014, there was a 10% increase in sepsis coding. This increase was not seen when clinical criteria alone was used to diagnose sepsis (+ 0.6%) [34]. This has been hypothesized as being due to upcoding, or the practice of prioritizing codes with higher reimbursement rates [35, 36].
Our results of an increasing prevalence of underlying pneumonia risk factors among hospitalized ACPI patients are in accordance with research showing that comorbidity prevalence is increasing among adults hospitalized for pneumonia from 2020 onward. In our study, the youngest age groups, < 50y, had a higher relative increase in risk factors for pneumonia compared to older age groups, also indicating a shift in the risk profile of ACPI hospitalizations after 2020. One study comparing non-COVID-19 pneumonia hospitalizations between 2016 and 2021 showed an increase in certain chronic conditions (immunosuppression, diabetes) present among patients hospitalized during the pandemic [37–39]. This may indicate a shift in the population to those with higher susceptibility to infection during a time when non-pharmaceutical interventions were practiced and non-COVID-19 respiratory pathogen circulation was lower [40, 41]. Some of these pneumonia hospitalizations represent bacterial infections secondary to Covid-19 diagnosis, which may contribute to the higher pneumonia incidence rate and prevalence of comorbidities in pandemic period data [42, 43].
Limitations
The database includes adults from all 50 US states; however, it is a convenience sample and subject to limitations regarding generalizability and representativeness. Adults ≥ 65y are underrepresented in the database as only enrollees with supplemental Medicare paid through private insurance are part of MarketScan data. Thus, patients who are ≥ 65y in this study may be healthier than the overall Medicare covered population and have higher income [44]. Claims data may also underrepresent outpatient risk factor diagnoses, particularly for diabetes, dementia, liver disease, and hypertension, that can be better captured through electronic medical record review [45–47]. Because underlying disease prevalence is dependent on a patient having a claim for a previous visit in the 3 years prior to an ACPI hospitalization, it is possible that an increasing comorbidity prevalence is due to an increase in medical claims per patient (i.e., better documentation of a risk factor). In our data, there was a 4% increase in unique outpatient records and 10% increase in inpatient records among hospitalized cases in 2020 compared to 2019. A study of matched COVID-19 infected and uninfected US Veterans showed that those who had a COVID-19 infection were more likely to have outpatient care records in the peri-infection period for up to 1 year post infection, which is consistent with trends in our study population [48]. Healthcare associated pneumonia was not explored in this analysis but may be of interest in future studies. Finally, enrollees needed at least 2 years of data within MarketScan to be included, which may create selection bias towards continuously insured adults.
The use of an expanded principal definition for ACPI or an any position definition for ACPI does carry some limitations. For studies that extend over a long period of time, the number of coding fields that are completed may increase with time, meaning adjustments may be necessary to avoid truncation bias in the early years of data [49, 50]. Future studies should explore whether the clinical characteristics and comorbidity distributions of ACPI hospitalizations satisfying the principal coding definition differs from those of ACPI satisfying the expanded principal definition or secondary only ACPI definitions.
This study is strengthened by the large size of the study population, ability to explore trends in individual risk factors for pneumococcal pneumonia, RSV, and severe influenza. Additional strengths of this study were the time period which allowed for examination of trends that covered the switch from ICD-9-CM to ICD-10-CM coding and pre- and post-COVID-19 period hospitalizations. Furthermore, the forward and reverse mapping of ICD codes to ensure complete capture of comorbidities reduced the impact of this coding shift on temporal trends. The incidence rates presented here are similar to those made using the same database with an overlapping time period [51]. Our findings that incidence rates calculated according to an expanded principal definition of ACPI are more stable than those defined by principal fields only align with previous research [21]. Our results are also consistent with publications on pneumococcal-specific risk factors among hospitalized all-cause pneumonia patients during the pre-pandemic period [52].
Conclusion
This study highlights the need for standardizing pneumonia claims definitions when assessing trends over time using a US claims database. Our results provide an example of how failure to account for pneumonia coding changes, particularly the shift from principal to secondary position codes, can overestimate the decline in pneumonia over time. The expanded ACPI definition, which incorporates both principal diagnostic codes for ACPI and principal codes for COPD, empyema, meningitis, septicemia, and respiratory failure in conjunction with a secondary code for ACPI, addresses a coding shift identified around 2017 in the MarketScan database while also reflecting the decrease in ACPI incidence. Using a principal ACPI hospitalization definition would exclude approximately 30% of hospitalizations that meet the expanded principal ACPI definition, thereby excluding a proportion of severe pneumonia cases that experience sepsis, respiratory arrest, and COPD during hospitalization. Additionally, the analysis revealed that patients hospitalized with pneumonia from 2020 to 2023 had higher prevalence of underlying comorbidities than those hospitalized in years prior.
Supplementary Information
Below is the link to the electronic supplementary material.
Acknowledgements
N/A.
Abbreviations
- ACIP
Advisory Committee on Immunization Practices
- ACPI
All-Cause Pneumonia and Influenza
- AHA
American Hospital Association
- COPD
Chronic Obstructive Pulmonary Disease
- ICD
International Classification of Diseases
- RSV
Respiratory Syncytial Virus
Author contributions
KC and NC designed the study. KC conducted the data analysis and drafted the first draft of the manuscript. All authors contributed to the revision of the manuscripts. All authors read and approved of the final version and agreed to its publication.
Funding
This study was funded by Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, USA.
Data availability
The data that support the findings of this study are available from Merative™ but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of Merative™. Merative and MarketScan are trademarks of Merative Corporation in the United States, other countries, or both.
Declarations
Ethical approval
Ethical approval was not required as part of this study of secondary, non-identifiable healthcare claims data.
Consent for publication
Consent for publication was not necessary for this study.
Competing interests
Kelsie Cassell, Salini Mohanty, Kelly Johnson, and Nicole Cossrow are employees of Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, USA who may own stock/or hold stock options in Merck & Co., Inc., Rahway, NJ, USA.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Citations
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Supplementary Materials
Data Availability Statement
The data that support the findings of this study are available from Merative™ but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of Merative™. Merative and MarketScan are trademarks of Merative Corporation in the United States, other countries, or both.