Influenza Symptoms
Influenza presents with abrupt onset of fever, cough, myalgia, headache, malaise, sore throat, and rhinitis, though clinical presentation varies significantly by age group. 1, 2
Classic Symptom Constellation in Adults
The hallmark presentation includes: 3, 1
- Sudden onset of high fever (typically 38-40°C)
- Nonproductive cough
- Myalgia (muscle aches throughout the body)
- Headache
- Malaise and fatigue
- Sore throat
- Rhinitis (nasal congestion and clear discharge)
- Chills or rigors
Less common symptoms include conjunctivitis, abdominal pain, nausea, vomiting, and diarrhea. 3
Age-Specific Presentations
Neonates and Young Infants
Fever may be the only presenting feature, but infants can also present with non-specific signs mimicking bacterial sepsis: 3, 2
- Pallor and floppiness (poor peripheral circulation, poor tone)
- Lethargy and poor feeding
- Episodes of apnea
- Irritability and appearing toxic
Infants and Children Under 2 Years
This age group presents differently from older children and adults: 3
- Fever as the sole presenting feature is common
- Gastrointestinal symptoms (diarrhea and vomiting) occur more frequently than in adults
- Febrile convulsions are positively associated with influenza A (up to 20% of hospitalized children experience febrile seizures) 3
- Otitis media is a common complication 3
Older Children and Adolescents
The clinical triad of cough, headache, and pharyngitis has 80% sensitivity and 78% specificity for influenza during epidemic periods: 3
- Sudden onset of high fever
- Chills
- Cough (initially nonproductive)
- Headache
- Sore throat
- Fatigue
- Nasal stuffiness
- Conjunctivitis
Temporal Course
The incubation period ranges from 1-4 days, with an average of 2 days. 3, 1, 2
Uncomplicated influenza typically resolves after 3-7 days, though cough and malaise can persist for more than 2 weeks. 3, 2 Fever tends to settle within 2-4 days, while a dry cough and clear nasal discharge last 1-2 weeks. 3
Contagious Period
- Adults are contagious from the day before symptoms begin through 5-6 days after onset 3, 2
- Children can be infectious for up to 10 days after symptom onset and may shed virus before symptoms appear 3, 2
- Severely immunocompromised persons can shed virus for weeks or months 3, 2
Critical Diagnostic Caveat
Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens based on symptoms alone. 3, 2 The sensitivity and specificity of clinical definitions vary depending on co-circulation of other respiratory viruses and local influenza activity levels. 3 In young children presenting to primary care during non-pandemic seasons, there are no specific clinical features that distinguish influenza from other winter viruses. 3
Studies show that fever and cough have sensitivities of 63-78% and specificities of 55-71% compared to viral culture. 3 In vaccinated older persons with chronic lung disease, cough was not predictive of influenza infection, though fever or feverishness was 68% sensitive and 54% specific. 3
Warning Signs for Severe Disease
Patients at higher risk for complications and severe presentations include: 2
- Children under 5 years (especially under 2 years)
- Adults over 65 years
- Pregnant women
- Individuals with chronic cardiopulmonary conditions
- Immunocompromised individuals
- Non-ambulant children with neuromuscular conditions