What are the symptoms of influenza?

Symptoms of Influenza

The most common symptoms of influenza are sudden onset of fever, cough, headache, muscle pain (myalgia), fatigue, sore throat, and nasal congestion or rhinitis. 1

Age-Specific Symptom Presentation

Adults and Older Children

• Sudden onset of high fever and chills (76-100%)
• Cough (93% in confirmed influenza cases vs 80% in non-influenza respiratory infections) 2
• Headache
• Sore throat
• Fatigue/malaise (51-75%)
• Muscle aches (myalgia)
• Nasal congestion/rhinitis (91% vs 81% in non-influenza) 2
• Conjunctivitis (26-50%)

The combination of cough and fever has a positive predictive value of 79% for influenza infection when the virus is circulating in the community 2.

Infants and Young Children (under 2 years)

• Fever (may be the only presenting symptom)
• Irritability
• Gastrointestinal symptoms (diarrhea and vomiting) more common than in adults
• Febrile seizures (reported in 6-20% of hospitalized children) 1
• Otitis media (common complication in approximately 25% of children under 5) 1

Neonates

• Non-specific signs resembling sepsis
• Pallor
• Floppiness (poor peripheral circulation, poor tone)
• Lethargy
• Poor feeding
• Episodes of apnea
• Fever (may be the only presenting feature) 1

Duration of Symptoms

• Uncomplicated influenza typically resolves after 3-7 days for most people 1
• Fever typically settles within 2-4 days 1
• Cough and malaise can persist for more than 2 weeks 1

Contagious Period

• Adults can be contagious from the day before symptoms begin through approximately 6 days after onset 1
• Children may be infectious before symptoms appear and for up to 10 days after symptom onset 1

Complications of Influenza

Respiratory Complications

• Primary viral pneumonia
• Secondary bacterial pneumonia (commonly caused by S. pneumoniae or S. aureus) 1
• Croup (often more severe when caused by influenza than by parainfluenza virus) 1
• Bronchiolitis (influenza is second only to RSV as a cause) 1
• Otitis media (most common bacterial superinfection in children) 1

Neurological Complications

• Encephalopathy or encephalitis
• Febrile seizures (particularly in children)
• Transverse myelitis 1

Other Complications

• Myositis (muscle inflammation)
• Myocarditis and pericarditis
• Reye syndrome (rare) 1
• Exacerbation of underlying medical conditions (e.g., cardiac or pulmonary disease) 1

High-Risk Groups

• Children under 5 years (especially those under 2 years)
• Adults 65 years and older
• Individuals with chronic medical conditions:
  • Asthma and other respiratory diseases
  • Cardiac disease
  • Immunocompromised states
  • Diabetes and other metabolic conditions
  • Renal disease
  • Neurological conditions
  • Non-ambulant individuals 1

Clinical Pearls

• The clinical diagnosis of influenza can be challenging without laboratory confirmation, as symptoms overlap with other respiratory infections 3
• When influenza is circulating in the community, the presence of both cough and fever within 48 hours of symptom onset is highly predictive of influenza 2
• The positive predictive value increases with higher body temperature 2
• Young children are less likely to report typical influenza symptoms and may present with fever alone or with non-specific symptoms 1
• Asymptomatic infection can occur, though its contribution to transmission is uncertain 1

The hallmark of influenza infection is the abrupt onset of symptoms, which distinguishes it from many other respiratory infections that tend to develop more gradually 4. This characteristic sudden onset, particularly of fever with respiratory symptoms, should raise clinical suspicion for influenza during the epidemic season.