Write a short note on COVID-19 and pandemic influenza (H1N1).

Covid-19

Aetiology:

• SARS-CoV-2, an enveloped RNA betacoronavirus, transmitted by respiratory droplets and aerosols, and by contact with contaminated surfaces. The spike (S) protein binds the ACE2 receptor, with priming by the host protease TMPRSS2
• ACE2 is expressed in respiratory epithelium, vascular endothelium, gut, heart and kidney — explaining the multisystem nature of the disease.

Pathophysiology of severe disease: an initial viral replicative phase is followed in some patients by a dysregulated host inflammatory response ("cytokine storm") with diffuse alveolar damage, and — distinctively — a prothrombotic endothelialitis causing micro- and macrovascular thrombosis.

Clinical features:

• Common: fever, dry cough, fatigue, myalgia, sore throat, headache, breathlessness, and anosmia and ageusia (loss of smell and taste) — relatively specific for COVID-19.
• Gastrointestinal: diarrhoea, nausea, anorexia.
• Severity spectrum: asymptomatic; mild (upper respiratory); moderate (pneumonia without hypoxia); severe (SpO2 < 90–94%, respiratory rate > 30, or > 50% lung involvement); critical (ARDS, shock, multi-organ failure).
• "Silent" or happy hypoxia — profound hypoxaemia with little subjective breathlessness, a distinctive and dangerous feature; pulse oximetry is essential and must not be replaced by clinical impression.
• Deterioration characteristically occurs in the second week, as the inflammatory phase begins.
• Thrombotic complications — pulmonary embolism, stroke, myocardial infarction, limb ischaemia — at a rate well above other respiratory infections.
• Other: myocarditis, acute kidney injury, Guillain–Barré syndrome, and in children multisystem inflammatory syndrome (MIS-C).
• Post-COVID (long COVID) — persistent fatigue, breathlessness, cognitive impairment ("brain fog"), palpitations and mood disturbance lasting months.
• COVID-associated mucormycosis — a devastating complication seen in large numbers in India during the second wave, driven by the combination of diabetes, corticosteroid use and hypoxia; presents with facial pain and swelling, nasal discharge, black eschar, orbital involvement and cerebral extension.

Investigations:

• RT-PCR on a nasopharyngeal swab (the reference standard)
• rapid antigen tests (specific but less sensitive)
• CBC showing lymphopenia with raised CRP, ferritin, D-dimer and LDH (all prognostic)
• chest X-ray and HRCT showing peripheral, bilateral, lower-zone ground-glass opacities
• arterial blood gas
• CT severity score.

Management:

• Mild disease — home isolation, symptomatic treatment, monitoring of oxygen saturation, and warning signs.
• Moderate to severe — oxygen therapy with awake prone positioning, escalating to high-flow nasal oxygen, non-invasive ventilation and lung-protective mechanical ventilation for ARDS.
• Corticosteroids (dexamethasone) — of proven mortality benefit in patients requiring oxygen, and harmful in those who do not (a crucial distinction that was widely ignored, contributing to the mucormycosis epidemic).
• Immunomodulators — tocilizumab and baricitinib in severe inflammatory disease.
• Antivirals — remdesivir and oral agents in selected patients early in the illness.
• Thromboprophylaxis with low-molecular-weight heparin for all hospitalised patients, with therapeutic anticoagulation for confirmed thrombosis.
• Avoid inappropriate antibiotics — bacterial co-infection is uncommon at presentation.
• Prevention: vaccination (the single most effective intervention), masks, ventilation, hand hygiene and isolation of cases.

Pandemic Influenza a (H1N1) — "swine Flu"

Aetiology: the influenza A (H1N1) pdm09 virus, a triple reassortant of swine, avian and human influenza genes, which caused the 2009 pandemic and now circulates as a seasonal strain with periodic outbreaks in India, particularly in winter and the post-monsoon period.

Clinical features:

• abrupt onset of fever, cough, sore throat, rhinorrhoea, severe myalgia and headache
• gastrointestinal symptoms (vomiting and diarrhoea) are more prominent than with seasonal influenza. Complications include primary viral pneumonia with rapid progression to ARDS, secondary bacterial pneumonia (pneumococcus and *Staphylococcus aureus*), myocarditis, encephalitis, myositis with rhabdomyolysis, and exacerbation of chronic disease.

High-risk groups: pregnant women (highest risk of severe disease and death), children under 5, adults over 65, obesity, and those with chronic lung, heart, liver, renal, neurological or metabolic disease, immunosuppression or malignancy.

India's categorisation (used for triage):

• Category A — mild illness, no testing, home care
• Category B — high-risk group or comorbidity, treat with oseltamivir without testing
• Category C — breathlessness, chest pain, drowsiness, hypotension, haemoptysis, cyanosis, or worsening of comorbidity — admit and test.

Investigations:

• RT-PCR on throat or nasopharyngeal swab (the confirmatory test)
• rapid antigen tests have limited sensitivity
• chest X-ray, CBC, and markers of severity.

Management:

• oseltamivir, started as early as possible — ideally within 48 hours of symptom onset, and given on clinical suspicion in high-risk and severe cases without waiting for test results
• oxygen and ventilatory support
• antibiotics for suspected bacterial co-infection
• supportive care.
• Zanamivir is an alternative.

Prevention: annual influenza vaccination — particularly for healthcare workers, pregnant women, the elderly, children and those with comorbidity; respiratory hygiene, hand washing, masks and isolation of cases.