What are the differences between influenza (flu) and COVID-19 in terms of causative viruses, transmission, symptoms, and management?

Key Differences Between Influenza and COVID-19

COVID-19 causes significantly more severe disease than influenza, with 2-3 times higher mortality, longer hospitalizations, more intensive care admissions, and greater risk of respiratory complications, despite sharing similar initial symptoms. 1, 2

Causative Viruses

• COVID-19 is caused by SARS-CoV-2, a novel betacoronavirus that uses ACE2 receptors to enter human cells, which are abundantly expressed in respiratory, gastrointestinal, and hepatic tissues 3
• Influenza is caused by influenza viruses (types A and B), which are orthomyxoviruses with different cellular entry mechanisms 4
• Both are single-stranded RNA viruses but belong to completely different viral families 3

Transmission Characteristics

• COVID-19 spreads primarily through respiratory droplets and close contact, with the virus remaining viable in aerosols for hours and on surfaces for days 3
• Asymptomatic transmission is a critical feature of COVID-19, with infected individuals shedding virus even without symptoms 3
• Influenza also transmits via respiratory droplets but has a shorter surface survival time 5
• Both viruses have similar modes of transmission, making clinical differentiation challenging without testing 5, 6

Incubation Period and Disease Timeline

• COVID-19 has an incubation period of 1-14 days (most commonly 3-7 days), with disease progression occurring approximately 5-8 days after symptom onset 3
• Viral shedding in COVID-19 continues beyond 10 days after symptom onset in severe cases 3
• Influenza typically has a shorter incubation period of 1-4 days 5

Clinical Symptoms: Key Distinguishing Features

Symptoms More Common in COVID-19:

• Gastrointestinal symptoms (diarrhea, nausea, vomiting) occur in 1-36% of COVID-19 patients but are less frequent in influenza 3, 6
• Neurological manifestations including confusion, altered mental status, and loss of taste/smell (anosmia/ageusia) are significantly more common in COVID-19, affecting 36.4% of all cases 7, 8, 6
• Dyspnea develops more frequently and severely in COVID-19 patients 1, 6

Symptoms More Common in Influenza:

• Upper respiratory symptoms (rhinorrhea, nasal congestion, sore throat) are more prominent in influenza 1, 6
• Fever occurs more consistently in influenza (present in most cases) compared to COVID-19 (58.6-77% of cases) 8, 6
• Vomiting and otorhinolaryngological symptoms are significantly more frequent in influenza 6

Shared Symptoms:

• Both cause fever, cough, fatigue, muscle aches, and headache, making clinical diagnosis without testing unreliable 5, 6

Disease Severity and Outcomes

COVID-19 Has Substantially Worse Outcomes:

• Mortality rate is 2.8-2.9 times higher than influenza (16.9% vs 5.8% in hospitalized patients) 2
• Hospital length of stay is 3.2 days longer on average 1
• ICU stay is 3.1 days longer 1
• Mechanical ventilation is required 2.3 times more frequently 1
• Intensive care admission occurs in 14.2% of COVID-19 hospitalizations with 88.1% mortality among those requiring invasive ventilation 3

Disease Severity Classification:

• COVID-19: 81% mild cases, 14% severe, 5% critical 3
• Severe COVID-19 is defined by oxygen saturation ≤93-94%, respiratory rate ≥30/min, or lung infiltrates >50% 7, 8

Complications

More Common in COVID-19:

• Pulmonary embolism occurs significantly more frequently 2
• Septic shock and hemorrhagic stroke are more common 2
• Acute respiratory distress syndrome (ARDS) develops more often and progresses more rapidly 3
• Multiorgan failure including acute kidney injury is more frequent 3
• Long-COVID syndrome affects 32.1-87.4% of patients with persistent symptoms beyond 4 weeks 7

More Common in Influenza:

• Myocardial infarction and atrial fibrillation occur more frequently in influenza patients 2

Laboratory Findings

COVID-19 Characteristics:

• Lymphopenia (low lymphocyte count) is more pronounced 3, 6
• Thrombocytopenia (low platelet count) is significantly more common 6
• Elevated transaminases (liver enzymes) occur more frequently 6
• Elevated inflammatory markers including C-reactive protein, lactate dehydrogenase, and D-dimer 3

Influenza Characteristics:

• Higher white blood cell count is more typical 6
• Elevated procalcitonin is significantly more common, suggesting higher rates of bacterial co-infection 6

Radiological Findings

COVID-19 Imaging:

• Ground-glass opacities with peripheral distribution are characteristic 3, 6
• Interlobular septal thickening is common 6
• Bilateral involvement with multiple small infiltrates progressing to diffuse ground-glass shadows 3

Influenza Imaging:

• Consolidations and linear opacities are more typical 6
• Less characteristic peripheral distribution compared to COVID-19 6

Risk Factors and Comorbidities

Higher Risk in COVID-19:

• Obesity is significantly more common (41.7% of hospitalized patients) 3, 1
• Diabetes mellitus (33.8% of hospitalized patients) 3, 1
• Hypertension (56.6% of hospitalized patients) 3, 2
• Dyslipidemia is more frequent 2
• Male sex confers higher risk (OR = 1.46) 1

Higher Risk in Influenza:

• Chronic respiratory diseases (COPD, asthma) are more common comorbidities 2, 6
• Heart failure is more frequent 2
• Cirrhosis and immunocompromised conditions are more common 2, 6

Pediatric Considerations

• Hospitalization rates are much lower for COVID-19 (1.4%) compared to influenza (19.5%) in children 2
• However, in-hospital mortality in adolescents (11-17 years) is 10 times higher for COVID-19 (1.1% vs 0.1%) 2
• Children under 5 years require intensive care more frequently with COVID-19 (2.3% vs 0.9%) 2
• Pediatric COVID-19 patients are more frequently obese or overweight 2

Diagnostic Approach

• RT-PCR testing is essential for definitive diagnosis of both infections, as clinical features alone cannot reliably distinguish between them 3, 5
• Sensitivity of RT-PCR for critically ill COVID-19 patients is unknown and may require multiple samples 3
• Co-infection with both viruses occurs and carries elevated risk for poor outcomes, necessitating testing for both pathogens in high-risk patients 5, 4
• Screen for influenza in all COVID-19 patients presenting during flu season, as co-infection affects 15.7% with poor outcomes (death/deterioration) 4

Management Differences

COVID-19 Management:

• Supportive care with oxygen therapy is the mainstay 3
• Corticosteroids may be considered but benefits and risks remain unclear in guidelines 3
• No specific antiviral is universally recommended, though various agents are being studied 3
• Anticoagulation is often required due to high thrombotic risk 3

Influenza Management:

• Oseltamivir (neuraminidase inhibitor) is the standard antiviral treatment 4
• Oseltamivir has NOT been shown effective for COVID-19 treatment 5
• Earlier initiation of specific antiviral therapy is possible with influenza 4

Critical Pitfall:

• Never assume influenza based on clinical presentation alone during COVID-19 circulation—both require specific testing as treatment differs fundamentally 5, 6
• Influenza antivirals are ineffective against COVID-19, making accurate diagnosis essential 5

Prevention Strategies

• Influenza vaccination is critical to reduce healthcare burden during COVID-19 circulation and should be promoted aggressively 5, 4
• Being unvaccinated for influenza is a significant risk factor for poor outcomes in co-infected patients 4
• COVID-19 vaccination is recommended for high-risk groups including older adults, immunocompromised patients, and those with comorbidities 7
• Infection control measures (hand hygiene, physical distancing, masks, ventilation) are essential for both viruses 7

Healthcare System Impact

• COVID-19 places substantially greater burden on healthcare systems with longer hospitalizations, higher ICU utilization, and greater resource requirements including ECMO 3, 1
• The combination of influenza season with ongoing COVID-19 transmission creates compounding stress on already strained healthcare infrastructure 5
• Healthcare workers face higher infection risk with COVID-19 due to aerosol-generating procedures, with over 3,000 infected in China early in the pandemic 3