What are the differences in symptoms between COVID-19 and influenza?

Symptom Differences Between COVID-19 and Influenza

COVID-19 and influenza share many overlapping symptoms, but COVID-19 is distinguished by a significantly higher frequency of loss of taste and smell, longer hospitalization duration, and greater risk of severe complications including ARDS, thromboembolism, and death. 1

Core Distinguishing Features

Loss of Taste and Smell (Most Specific)

• Anosmia (loss of smell) occurs in 85.6% of COVID-19 patients 1
• Ageusia (loss of taste) occurs in 88.8% of COVID-19 patients 1
• These symptoms are significantly more common in COVID-19 than influenza and serve as stronger predictors of COVID-19 than self-reported fever 1

Common Overlapping Symptoms

Both diseases present with similar respiratory symptoms, making clinical differentiation challenging without testing:

COVID-19 symptom frequencies: 1

• Fever: 92.8%
• Dry cough: 69.8%
• Fatigue: 38.5%
• Myalgia (muscle pain): 27.7%
• Headache: 7.2%
• Diarrhea: 6.1%
• Sore throat: 5.1%
• Rhinorrhea (runny nose): 4.0%

Influenza characteristics: 2

• Children infected with influenza are more likely to exhibit symptoms than adults (only 6.6% asymptomatic)
• Upper respiratory symptoms are more prominent in influenza 3
• Asthma and COPD exacerbations are more common with influenza than COVID-19 4

Disease Severity and Clinical Course

COVID-19 Severity Spectrum

Mild cases (81%): Non-specific symptoms including fever, cough, sore throat, and malaise without dyspnea or abnormal chest imaging 2, 1

Severe cases (14%): 1

• Respiratory rate ≥30 breaths/min
• Oxygen saturation ≤93%
• Lung infiltrates >50% within 24-48 hours

Critical cases (5%): Respiratory failure, septic shock, and/or multiple organ dysfunction 1

Influenza Severity

• Hospitalization rates highest in children <5 years, particularly infants <6 months 2
• 17.8% develop pneumonia, 21.6% require ICU admission, 5.3% require mechanical ventilation 2
• In-hospital mortality: 0.5% for influenza vs 0.7% for COVID-19 in pediatric populations 2

Complications: COVID-19 vs Influenza

COVID-19 Has Significantly Higher Risk For:

Respiratory complications: 4

• ARDS: 18.6 times higher risk than influenza
• Pulmonary embolism: 2.1 times higher risk

Cardiovascular complications: 4

• Myocarditis: 2.56 times higher risk
• Deep vein thrombosis: 2.81 times higher risk

Neurologic complications: 4

• Intracranial hemorrhage: 2.85 times higher risk

Other complications: 4

• Acute hepatitis/liver failure: 3.13 times higher risk
• Bacteremia: 2.46 times higher risk
• Septic shock and multiple organ dysfunction 2

Influenza Has Higher Risk For:

• Asthma exacerbations: COVID-19 has 0.27 times the risk of influenza 4
• COPD exacerbations: COVID-19 has 0.37 times the risk of influenza 4
• Bacterial co-infections: 11-35% for influenza vs 5.1% for COVID-19 5

Clinical Outcomes

Mortality

• COVID-19 in-hospital mortality: 21.0% 4
• Influenza in-hospital mortality: 3.8% 4
• COVID-19 mortality is more than 5 times higher than influenza 4
• Age-standardized mortality ratio: 2.82 for COVID-19 vs influenza 6

Healthcare Resource Utilization

COVID-19 patients require: 3

• Longer hospital stays (3.2 days longer on average)
• Longer ICU stays (3.1 days longer on average)
• More frequent mechanical ventilation (2.3 times higher)

Special Populations

Pediatric Considerations

COVID-19 in children: 1

• May present with fever, fatigue, cough, nasal congestion, runny nose, expectoration, diarrhea, and headache
• Often have milder clinical presentations than adults
• Asymptomatic infection is more common in children

Influenza in children: 2

• Higher hospitalization rates overall (19.5% of influenza hospitalizations vs 1.4% for COVID-19)
• However, children <5 years with COVID-19 require ICU support more frequently (2.3% vs 0.9%)
• Adolescents (11-17 years) have 10 times higher in-hospital mortality with COVID-19 than influenza (1.1% vs 0.1%) 6

High-Risk Groups

COVID-19 risk factors: 2

• Age >65 years
• Cardiovascular disease, diabetes, cancer, COPD, hypertension
• Male gender associated with more severe disease
• Obesity more common in COVID-19 patients 6, 3

Influenza risk factors: 2

• Children <5 years (especially <6 months)
• Underlying neurologic conditions
• Chronic respiratory disease more common in influenza patients 6

Clinical Pitfalls

Key Diagnostic Challenges

• Both diseases can present identically with fever, cough, headache, muscle aches, and fatigue 7
• Clinical diagnosis without testing is impaired due to overlapping presentations 7
• Co-infection is possible and can worsen outcomes 2, 7
• Asymptomatic transmission occurs with COVID-19, making symptom-based screening alone inadequate 5

Testing Recommendations

• Nasopharyngeal swab RT-PCR remains the gold standard for COVID-19 diagnosis (sensitivity 60-78%) 1
• If initial RT-PCR is negative but symptoms persist, repeat testing should be considered due to false negatives 1
• Testing is essential for definitive diagnosis given symptom overlap 7, 8