Differentiating COVID-19 From Other Common Viral Infections

Although these clinical pointers and scoring tools are invaluable, confirmatory testing (e.g., RT-PCR or rapid antigen testing) help steer the diagnosis in the correct direction.

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Differentiating COVID-19 from other common viral infections in a clinical setting can be challenging because many viruses produce overlapping symptoms. However, several clinical clues and objective findings may help towards the correct diagnosis even before confirmatory diagnostics are available.

Dr Deepesh G Aggarwal, Consultant Physician & HoD- Dept of Critical Care Medicine, Saifee Hospital, Mumbai differentiates COVID 19 from other common viral infections.

Key Clinical Differences

1. Symptom Profile

COVID-19:

• Anosmia and Ageusia: Loss of smell and taste are well‐recognised features in COVID-19 and are much less common with other respiratory viruses. It was more so in the initial wave. It is less common in the current variant.
• Subtle Onset and Lower Respiratory Predilection: Many COVID-19 patients report a more insidious onset of symptoms with a tendency toward lower respiratory tract involvement.

Extra-Pulmonary Features:  Patients may more commonly report these features, especially after universal vaccination:

1. Gastrointestinal symptoms (e.g., anorexia, nausea, vomiting, diarrhea, Abdominal Discomfort ranging from Mild cramping to symptoms mimicking Acute abdomen) 

2. Neurological Symptoms (e.g. headache-may be very severe and disproportionate, Dizziness, “brain fog”, Meningitis, Encephalopathy or frank Encephalitis, Stroke, Seizures; neuropathies: Isolated Cranial Neuropathies, peripheral Neuropathies & Guillain–Barré syndrome. 

3. Long COVID: lingering issues such as chronic fatigue, sleep disturbances, and mood changes.

Other Viral Infections:

• Influenza/RSV: Often present with a more abrupt onset. Influenza tends to show high fever, more prominent rhinorrhea, productive cough (characterised by purulent sputum in bacterial infections), and myalgia.
• Dengue or Other Arboviruses: These usually have distinct systemic features like severe headache, retro-orbital pain, rash, and significant myalgia. Thrombocytopenia is also a key laboratory finding that differentiates them from COVID-19.

2. Laboratory Findings and Imaging

COVID-19:

• Lymphopenia: A reduced lymphocyte count (often <1000 cells/mm³) is frequently observed in COVID-19 patients.
• Imaging: Chest CT may reveal bilateral ground-glass opacities with peripheral (sub-Pleural) distribution—findings that can help distinguish it from other viral pneumonias that may show more lobar or bronchopneumonia patterns.

Other Viral Infections:

• Influenza/RSV: While lymphopenia can occur, the presence of rhinorrhea and productive cough tends to be more pronounced. Imaging may not show the classic peripheral (sub-Pleural) ground-glass pattern seen with COVID-19.
• Dengue: Laboratory tests typically reveal thrombocytopenia and sometimes atypical lymphocytes, which are not expected in COVID-19.

3. Clinical Scoring Systems

While history and physical examination remain the main stay of diagnosis, several studies have proposed scoring systems to support clinical impression:

• A study developed a Flu-RSV/COVID score incorporating predictors like age (>50 years), underlying disease, rhinorrhea, productive sputum, and lymphocyte count <1000 cells/mm. This score demonstrated high sensitivity and specificity in differentiating influenza/RSV from COVID-19.
• Similarly, for differentiating dengue from COVID-19, a separate score was proposed that included factors such as headache, myalgia, absence of cough, thrombocytopenia (platelet count <150,000/mm³), and lymphocyte count <1000 cells/mm.