What are the signs and symptoms of influenza?

Signs and Symptoms of Influenza

Influenza presents with abrupt onset of fever, cough, myalgia, headache, and malaise, typically beginning 1-4 days after exposure, with symptoms varying significantly by age group. 1, 2

Classic Presentation in Adults and Older Children

The hallmark of influenza infection is the sudden onset of constitutional and respiratory symptoms occurring together rather than sequentially. 1, 2

Key symptoms include:

• Fever (often high-grade, >100°F) with chills or rigors 1, 2
• Nonproductive cough (dry cough without sputum) 1, 2
• Myalgia (muscle aches affecting multiple body regions) 1, 2
• Headache (often severe) 1, 2
• Severe malaise and fatigue 1, 2
• Sore throat 1, 2
• Rhinitis (nasal congestion and rhinorrhea) 1, 2

The combination of cough and fever together has 79% positive predictive value for influenza when the virus is circulating in the community. 3 In children aged 6 years and older, the triad of cough, headache, and pharyngitis has 80% sensitivity and 78% specificity for influenza. 1

Age-Specific Presentations

Neonates and Young Infants

Infants often present with non-specific signs that mimic bacterial sepsis, making influenza diagnosis particularly challenging in this age group. 1, 2

• Fever may be the only presenting feature 1, 2
• Pallor and floppiness (poor peripheral circulation, decreased tone) 1, 2
• Lethargy and poor feeding 1, 2
• Episodes of apnea 1, 2
• Irritability and appearing toxic 1, 2

Infants and Children Under 2 Years

Young children are less likely to report typical influenza symptoms and more likely to present with gastrointestinal manifestations. 1

• Fever (often the only initial symptom) 1, 2
• Gastrointestinal symptoms (diarrhea and vomiting are common) 1, 2
• Irritability 1, 2
• Febrile seizures (particularly repeated convulsions are positively associated with influenza A, occurring in up to 20% of hospitalized children) 1
• Otitis media (occurs in approximately 25% of children under 5 with influenza) 1

Older Children (2-12 Years)

The presentation in older children more closely resembles adults but with some distinctive features. 1, 2

• Sudden onset of high fever 1, 2
• Cough, headache, and pharyngitis (the classic triad) 1
• Sore throat 1, 2
• Fatigue 1, 2
• Nasal stuffiness and clear nasal discharge 1
• Conjunctivitis 1, 2
• Nausea and vomiting (more common than in adults) 1

Timeline and Contagiousness

The incubation period averages 2 days (range 1-4 days). 1, 2

Adults are contagious from the day before symptoms begin through 5-6 days after illness onset. 1, 2 Children can be infectious for up to 10 days after symptom onset. 1, 2 This longer period of viral shedding in children has important implications for infection control in households and schools. 1

Immunocompromised individuals may shed virus for weeks or months, representing a prolonged transmission risk. 1, 2

Duration and Resolution

Uncomplicated influenza typically resolves after 3-7 days for most people. 1, 2 However, cough and malaise commonly persist for more than 2 weeks even after other symptoms resolve. 1, 2

In children, fever typically settles within 2-4 days, though a dry cough and clear nasal discharge may last 1-2 weeks. 1

Complications and Warning Signs

Influenza can lead to serious complications that require immediate medical attention. 1, 2

Common complications include:

• Secondary bacterial pneumonia (most common serious complication, occurring in 20-38% of severe cases requiring intensive care) 1, 2
• Primary influenza viral pneumonia 1, 2
• Exacerbation of underlying chronic conditions (cardiac, pulmonary) 1, 2
• Sinusitis and otitis media 1

Severe but less common complications:

• Encephalopathy or encephalitis (presenting as seizures or altered mental status) 1, 2
• Acute respiratory distress syndrome 1, 2
• Myositis (muscle inflammation) 1
• Myocarditis and pericarditis 1
• Transverse myelitis 1
• Reye syndrome (particularly with aspirin use in children) 1

Critical Diagnostic Limitations

A crucial caveat: respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens based on symptoms alone. 1, 2 Respiratory syncytial virus, parainfluenza, adenovirus, and metapneumovirus present with nearly identical clinical features. 4

The sensitivity and specificity of clinical definitions for influenza-like illness vary depending on:

• The degree of co-circulation of other respiratory pathogens 1
• The level of influenza activity in the community 1
• The specific viral strains circulating 1

When influenza is known to be circulating in the community, the combination of cough and fever within 48 hours of symptom onset has the highest predictive value for influenza infection. 3 However, laboratory confirmation remains valuable when it will change management decisions, particularly in hospitalized patients or those at high risk for complications. 5

High-Risk Populations

Certain groups experience more severe symptoms and prolonged illness: 1, 2

• Children under 5 years (especially under 2 years) 1, 2
• Adults over 65 years 1
• Pregnant women 2
• Individuals with chronic cardiac or respiratory disease 1, 2
• Immunocompromised individuals 1, 2
• Non-ambulant children 1, 2

These populations require heightened clinical suspicion and lower thresholds for antiviral treatment and hospitalization. 2, 5