From the Guidelines
Severe influenza infection is defined by the presence of severe viral pneumonia, respiratory failure, or shock, often requiring intensive care unit admission and resulting in high mortality rates.
Objective Criteria
The following objective criteria can be used to define severe influenza infection:
- Respiratory failure: requiring mechanical ventilation or resulting in severe hypoxemia
- Severe viral pneumonia: characterized by diffuse viral pneumonitis, often leading to respiratory failure
- Shock: resulting from severe influenza infection, often requiring vasopressor support
- Intensive care unit admission: requiring close monitoring and supportive care due to severe illness
- High mortality rates: resulting from severe influenza infection, particularly in high-risk populations such as older adults, young children, and those with underlying comorbidities 1.
High-Risk Populations
Certain populations are at increased risk for severe influenza infection, including:
- Older adults: aged 65 years and older
- Young children: aged less than 5 years
- Pregnant and postpartum women: within 2 weeks of delivery
- Individuals with underlying comorbidities: such as asthma, chronic obstructive pulmonary disease (COPD), cardiac disease, and immunosuppression 1.
Clinical Presentation
The clinical presentation of severe influenza infection can vary, but often includes:
- Fever: high and persistent
- Cough: severe and productive
- Shortness of breath: severe and progressive
- Chest pain: severe and persistent
- Confusion: or altered mental status 1.
From the Research
Objective Criteria for Defining Severe Influenza Infection
The definition of severe influenza infection can be based on several objective criteria, including:
- Respiratory failure requiring intensive care support 2
- Bacterial and fungal coinfections in severe influenza pneumonia, which are associated with worse outcomes 2, 3
- Acute respiratory distress syndrome (ARDS) 3, 4
- Organ dysfunction and septic shock 3
- Need for invasive or non-invasive mechanical ventilation 5
- Presence of pulmonary infiltrates on imaging, heart rate ≥110 beats/minute, oxygen saturation <93%, and respiratory rate >24 breaths/minute 5
Biomarkers and Clinical Parameters
Several biomarkers and clinical parameters can be used to identify patients with severe influenza infection, including:
- Procalcitonin, ferritin, human leukocyte antigen-DR, mid-regional proadrenomedullin, and lactate 3
- Vital signs, such as heart rate, respiratory rate, and oxygen saturation 5
- Laboratory findings, such as white blood cell count and presence of pulmonary infiltrates on imaging 5
- Clinical interventions, such as need for mechanical ventilation and vasopressor support 5
Scales and Case Definitions
Several scales and case definitions have been developed to differentiate severe and non-severe lower respiratory tract illness in patients hospitalized with influenza, including:
- The Influenza Disease Evaluation and Assessment of Severity (IDEAS) scale 5
- The severe acute respiratory illness (SARI) case definition 6
- The clinical pneumonia case definitions, such as the Integrated Management of Childhood Illnesses (IMCI) and the Integrated Management of Adolescent and Adult Illnesses (IMAI) 6