What are the typical symptoms and clinical presentation of influenza, including common findings in adults, children, and high‑risk groups?

Symptoms of Influenza

Influenza presents with the abrupt onset of fever, cough, myalgia, headache, and malaise—this constellation of symptoms appearing suddenly within hours is the hallmark that distinguishes influenza from other respiratory infections. 1

Core Clinical Presentation in Adults

The typical influenza syndrome is characterized by:

• Fever (≥38°C in adults) occurring in 68-100% of confirmed cases, typically lasting 2-4 days with peak severity in the first 24-48 hours 1, 2
• Cough (nonproductive initially) present in 80-93% of patients, often persisting 1-2 weeks after other symptoms resolve 1, 3
• Myalgia (muscle aches) and severe malaise appearing abruptly at onset 1, 4
• Headache developing suddenly and typically resolving within 3-7 days alongside constitutional symptoms 1, 5, 6
• Sore throat and rhinitis (nasal congestion/discharge) in 20-91% of cases 1, 3

The combination of cough plus fever has a 79% positive predictive value for laboratory-confirmed influenza when the virus is circulating in the community, making this the most clinically useful diagnostic criterion 3.

Clinical Presentation in Children

Pediatric influenza presentations vary significantly by age:

Infants and Young Children (<2 years)

• May present with non-specific sepsis-like symptoms including lethargy, poor feeding, irritability, and apnea rather than classic respiratory symptoms 1, 7
• Fever threshold is higher (≥38.5°C) compared to adults 1
• More likely to appear toxic with pallor and floppiness 1, 7
• Gastrointestinal symptoms (diarrhea, vomiting, abdominal pain) occur in 14-57% of pediatric cases, significantly more common than in adults 1

Older Children (>2 years)

• Present similarly to adults with sudden onset of high fever, chills, cough, headache, sore throat, and fatigue 1, 7
• Febrile convulsions are positively associated with influenza A, particularly recurrent seizures 1
• Otitis media complicates approximately 25% of cases in children under 5 years 1, 7
• Conjunctivitis and nasal stuffiness are common additional features 1, 7

High-Risk Groups and Severe Presentations

Certain populations warrant heightened clinical vigilance:

Immunocompromised Patients

• May lack adequate febrile response despite active infection 1
• Can shed virus for weeks to months rather than the typical 5-10 days 1, 7, 2
• Higher risk of progression to severe complications 2

Elderly Patients (≥65 years)

• Often experience longer symptom duration and higher complication rates 2
• May present with atypical symptoms or blunted fever response 1

Pregnant Women and Chronic Disease Patients

• At substantially increased risk for severe disease requiring hospitalization 7
• More likely to develop primary viral pneumonia or bacterial superinfection 1, 7

Warning Signs of Complications

Fever persisting beyond 3-4 days should trigger immediate reassessment for bacterial superinfection, as this timeline exceeds typical uncomplicated influenza 7, 2. Additional red flags include:

• Biphasic fever pattern (initial improvement followed by recurrence) suggesting secondary bacterial pneumonia 7, 2
• Respiratory distress: tachypnea (>40 breaths/min in children, >50 in infants), chest retractions, dyspnea, or cyanosis 1, 7
• Altered mental status: lethargy, confusion, decreased consciousness, or excessive drowsiness 1, 7, 5
• Blood-tinged sputum or severe chest pain 2
• Persistent vomiting >24 hours in children 7

Infectious Period and Transmission

• Incubation period: 1-4 days (average 2 days) 1, 7
• Adults: contagious from 1 day before symptoms through 5-6 days after onset 1, 7
• Children: can remain infectious for up to 10 days after symptom onset, considerably longer than adults 7, 2
• Viral shedding may begin several days before clinical illness appears 7

Clinical Pearls

Respiratory illness caused by influenza cannot be reliably distinguished from other respiratory pathogens based on symptoms alone—the positive predictive value of clinical definitions ranges from only 63-78% even during peak influenza season 1, 7. However, when influenza is known to be circulating in the community, the combination of cough plus fever within 48 hours of onset provides the strongest clinical prediction 3.

Uncomplicated influenza typically resolves within 3-7 days, though cough and malaise commonly persist for more than 2 weeks after fever resolution 1, 7, 2. This prolonged cough should not be confused with ongoing infection or bacterial superinfection unless accompanied by other warning signs 2.

Gastrointestinal symptoms are significantly more common in children than adults and should not exclude the diagnosis of influenza, particularly in young children where diarrhea and vomiting may be prominent features 1.