COVID-19 & Post-COVID Syndrome
Medicine · Infectious Disease · lean revision notes
COVID-19 & Post-COVID Syndrome
Coronavirus disease 2019 (COVID-19) is an acute respiratory illness caused by the novel betacoronavirus SARS-CoV-2. Though the pandemic phase has subsided, NEET PG continues to test its core mechanisms — ACE2 entry, the cytokine storm, WHO severity grading, evidence-based therapy (dexamethasone, remdesivir, anticoagulation) and the increasingly examined entity of post-COVID/long COVID syndrome.
Etiology & Virology
SARS-CoV-2 is an enveloped, positive-sense single-stranded RNA virus of the family Coronaviridae, genus Betacoronavirus (Sarbecovirus subgenus). It is the third highly pathogenic human coronavirus after SARS-CoV-1 (2002) and MERS-CoV (2012).
Key structural proteins (mnemonic SHEN is informal; remember S, E, M, N):
| Protein | Role / Exam point |
|---|---|
| Spike (S) | Binds ACE2; cleaved by host TMPRSS2; target of most vaccines & monoclonals |
| Envelope (E) | Smallest; viral assembly & release |
| Membrane (M) | Most abundant; shapes virion |
| Nucleocapsid (N) | Binds RNA genome; target of rapid antigen tests |
The spike protein has two subunits: S1 (contains the receptor-binding domain, RBD) and S2 (mediates membrane fusion). Variants of concern (Alpha, Beta, Gamma, Delta, Omicron) carry RBD mutations altering transmissibility and immune escape; Omicron and its sublineages dominate current circulation.
High-yield: The cellular receptor is ACE2 (angiotensin-converting enzyme 2) and the priming protease is TMPRSS2. ACE2 is highly expressed in type II pneumocytes, enterocytes, vascular endothelium, renal tubular cells and cardiac myocytes — explaining the multi-organ involvement.
Pathophysiology
The disease classically evolves in three overlapping phases:
Stage I (early infection / viral replication) → Stage II (pulmonary phase) → Stage III (hyperinflammation / cytokine storm)
- Viral entry: Spike RBD binds ACE2; TMPRSS2 cleaves spike to permit membrane fusion and RNA entry. Binding of the virus also downregulates ACE2, leaving unopposed angiotensin II (pro-inflammatory, vasoconstrictive, pro-fibrotic) — a mechanism implicated in lung injury.
- Innate immune response & cytokine storm: Dysregulated release of IL-6, IL-1β, TNF-α, IL-2, GM-CSF and ferritin drives diffuse alveolar damage. Markedly elevated IL-6 correlates with severity.
- Immunothrombosis: Endothelial injury, complement activation, platelet activation and a hypercoagulable state produce microthrombi and venous thromboembolism. Pulmonary microvascular thrombosis is a hallmark at autopsy.
- Diffuse alveolar damage (DAD): Hyaline membranes, type II pneumocyte hyperplasia and later fibrosis — the histological basis of ARDS.
High-yield: COVID-19 ARDS is characterised by profound hypoxaemia with relatively preserved lung compliance early on ("happy/silent hypoxia") — patients have low SpO₂ disproportionate to their dyspnoea due to impaired hypoxic pulmonary vasoconstriction and V/Q mismatch.
Clinical Features
- Incubation period: 2–14 days (median ~4–5 days; shorter, ~3 days, for Omicron).
- Common: fever, dry cough, fatigue, myalgia, sore throat, headache.
- Highly specific: anosmia (loss of smell) and ageusia (loss of taste) — classic early COVID features, less common with Omicron.
- Respiratory: dyspnoea, hypoxia, ARDS (usually day 7–10).
- GI: diarrhoea, nausea, anorexia (faecal viral shedding occurs).
- Dermatologic: "COVID toes" (pernio/chilblain-like acral lesions), maculopapular rash, urticaria.
- Thrombotic: DVT, pulmonary embolism, stroke, acute coronary events.
In children: Multisystem Inflammatory Syndrome in Children (MIS-C) — Kawasaki-like illness with fever, shock, raised inflammatory markers and cardiac dysfunction occurring 2–6 weeks after infection.
WHO Severity Classification
This is a frequently tested grid. Learn the SpO₂ and respiratory-rate cut-offs precisely.
| Severity | Defining features |
|---|---|
| Mild | Symptomatic, meets case definition; no pneumonia or hypoxia; SpO₂ ≥ 94% on room air |
| Moderate | Clinical/radiological signs of pneumonia but SpO₂ ≥ 90%; RR < 30/min |
| Severe | SpO₂ < 90% on room air; RR > 30/min; or signs of severe respiratory distress |
| Critical | ARDS, sepsis, septic shock, or need for mechanical ventilation / vasopressors |
High-yield: The single most important bedside cut-off is SpO₂ < 90% (room air) or RR > 30/min = severe disease → admit, oxygen, dexamethasone. (Indian guidelines often use SpO₂ ≤ 93% as the threshold to start steroids/oxygen.)
Diagnosis & Investigations
Investigation of choice (confirmatory): RT-PCR for SARS-CoV-2 RNA from a nasopharyngeal/oropharyngeal swab — detects E gene (screening) and confirms with RdRp/N/ORF1ab genes.
| Test | Detects | Use / Caveat |
|---|---|---|
| RT-PCR | Viral RNA | Gold standard; highest sensitivity |
| Rapid antigen (RAT) | N protein | Fast, point-of-care; low sensitivity — a positive confirms, a negative does not exclude |
| IgM/IgG serology | Antibodies | Epidemiology / past infection, not acute diagnosis |
| CBNAAT / TrueNat | RNA | Cartridge-based, used in India |
Supportive bloods reflecting severity / poor prognosis:
- Lymphopenia (hallmark), neutrophilia, high N/L ratio.
- Raised D-dimer (thrombosis/prognosis), ferritin, CRP, LDH, IL-6.
- Raised troponin and NT-proBNP (myocardial involvement).
High-yield: Lymphopenia + raised D-dimer + raised ferritin + raised LDH is the classic "bad-prognosis" quartet. A rising D-dimer should trigger evaluation for thromboembolism.
Imaging
HRCT chest is the most sensitive imaging modality. Classic findings:
- Bilateral, peripheral, posterior, lower-lobe ground-glass opacities (GGO) — the signature.
- Crazy-paving (GGO + interlobular septal thickening).
- Reverse halo / atoll sign (organising pneumonia pattern).
- Later: consolidation, traction bronchiectasis, fibrosis.
CO-RADS is the standardised CT reporting scheme (1 = very low to 5 = very high probability of COVID). A CT severity score (CTSS) quantifies lobar involvement (e.g., /25), guiding prognosis.
High-yield: Peripheral, bilateral, subpleural ground-glass opacities sparing the central regions is the textbook CT description. Crazy-paving = GGO + septal thickening.
Management
Treatment is stage- and severity-tailored. The two interventions with the strongest mortality evidence are dexamethasone (in those needing oxygen) and anticoagulation prophylaxis.
Stepwise approach
Mild (home care) → symptomatic treatment (paracetamol, hydration, isolation), monitor SpO₂. Consider oral antivirals in high-risk patients.
Moderate (oxygen, ward) → supplemental O₂ to keep SpO₂ 92–96%, dexamethasone, prophylactic anticoagulation, consider remdesivir early.
Severe/Critical (ICU) → high-flow nasal O₂/NIV/intubation, awake proning, dexamethasone, add immunomodulator (tocilizumab or baricitinib) if escalating, therapeutic vs prophylactic anticoagulation per phenotype, manage ARDS with lung-protective ventilation (6 mL/kg, plateau < 30 cm H₂O).
Key drugs and the evidence
| Drug | Class / target | Evidence & use |
|---|---|---|
| Dexamethasone | Corticosteroid | RECOVERY trial — mortality benefit in patients needing oxygen or ventilation; 6 mg OD × up to 10 days. Harmful if no oxygen requirement. |
| Remdesivir | RNA-dependent RNA polymerase inhibitor | Shortens recovery time (ACTT-1); best early, in moderate disease needing low-flow O₂; not for ventilated/ECMO patients |
| Tocilizumab | Anti–IL-6 receptor mAb | Mortality benefit (RECOVERY/REMAP-CAP) in severe disease with rising CRP + needing O₂/support |
| Baricitinib | JAK 1/2 inhibitor | Alternative immunomodulator; benefit in hospitalised hypoxic patients |
| Nirmatrelvir–ritonavir (Paxlovid) | Protease (Mpro) inhibitor | Oral; early treatment in high-risk outpatients to prevent progression |
| Molnupiravir | Mutagenic nucleoside analogue | Oral antiviral; modest benefit, used in high-risk outpatients |
| LMWH / heparin | Anticoagulant | Prophylaxis for all admitted patients (unless contraindicated) |
High-yield: Dexamethasone helps ONLY those on oxygen/ventilation and may HARM those without hypoxia — this dichotomy is a favourite MCQ. Remdesivir = early/moderate disease; steroids/tocilizumab = the inflammatory phase.
High-yield: Hydroxychloroquine, lopinavir-ritonavir, azithromycin, ivermectin and convalescent plasma showed NO mortality benefit in large RCTs and are not recommended — also commonly tested as the "wrong" answer.
Prevention
- Vaccines: mRNA (BNT162b2, mRNA-1273), viral-vector (ChAdOx1/Covishield, Sputnik), inactivated whole-virion (Covaxin/BBV152), protein-subunit (Corbevax/NVX). Most target the spike protein.
- Non-pharmacological: masking, hand hygiene, ventilation, distancing.
Complications
- Pulmonary: ARDS, secondary bacterial/fungal pneumonia, pulmonary fibrosis, pneumothorax.
- Thrombotic: PE, DVT, arterial thrombosis, stroke, MI.
- Cardiac: myocarditis, arrhythmias, heart failure.
- Renal: acute kidney injury (poor prognostic marker).
- Neurological: encephalopathy, Guillain-Barré syndrome, stroke.
- Secondary infections / superinfections: notably COVID-associated mucormycosis (CAM, "black fungus") — linked to diabetes, indiscriminate steroid use, hypoxia and high serum iron; Rhizopus is the usual agent; managed with liposomal amphotericin B + surgical debridement + glycaemic control.
- MIS-C (children) and MIS-A (adults).
High-yield: Mucormycosis surged with COVID owing to the triad of uncontrolled diabetes + corticosteroid overuse + hypoxia (low pH, high free iron) — a recurring exam theme during the Indian Delta wave.
Post-COVID / Long COVID Syndrome
Definition (WHO): Persistence or new onset of symptoms ≥ 3 months from the onset of acute COVID-19, lasting at least 2 months, not explained by an alternative diagnosis. (US/CDC often uses a ≥ 4-week cut-off for "post-acute sequelae of SARS-CoV-2", PASC.)
Proposed mechanisms: persistent viral antigen/reservoirs, autoimmunity, endothelial dysfunction & microclots, dysautonomia, and mitochondrial dysfunction.
| Domain | Manifestations |
|---|---|
| Constitutional | Persistent fatigue (most common), post-exertional malaise, low-grade fever |
| Neurocognitive | "Brain fog", impaired concentration/memory, headache, sleep disturbance |
| Cardiovascular / autonomic | POTS and other dysautonomia, palpitations, chest pain |
| Respiratory | Breathlessness, persistent cough, reduced DLCO, fibrotic changes |
| Other | Anosmia/dysgeusia, hair loss (telogen effluvium), arthralgia, anxiety/depression, PTSD |
Management is multidisciplinary and largely supportive: graded exercise/pacing, pulmonary rehabilitation, treating dysautonomia/POTS (salt, fluids, beta-blockers), psychological support and screening for organ-specific sequelae.
High-yield: The single most common long-COVID symptom is fatigue, followed by dyspnoea and cognitive "brain fog". POTS (postural orthostatic tachycardia syndrome) is the classic dysautonomic manifestation tested in NEET PG.
Key Differentials
- Influenza / other viral pneumonias — overlapping flu-like illness; co-testing recommended in season.
- Bacterial community-acquired pneumonia — lobar consolidation, higher procalcitonin.
- Atypical pneumonia (Mycoplasma, Legionella, Chlamydophila).
- Pulmonary embolism — can coexist; consider with sudden hypoxia + raised D-dimer.
- Acute heart failure / cardiogenic pulmonary oedema.
- Hypersensitivity pneumonitis / organising pneumonia — radiological mimics of GGO.
Recently asked / exam angle
- Receptor & protease: ACE2 + TMPRSS2 — a near-guaranteed one-liner.
- RECOVERY trial: dexamethasone reduces mortality only in oxygen-requiring patients — classic single-best-answer.
- WHO severity cut-offs: SpO₂ < 90% / RR > 30 = severe; matching scenarios to mild/moderate/severe/critical.
- CT pattern: peripheral bilateral ground-glass opacities, crazy-paving, reverse-halo sign.
- Prognostic labs: lymphopenia, raised D-dimer/ferritin/LDH/CRP/IL-6.
- Mucormycosis triad (diabetes + steroids + hypoxia) and amphotericin B management.
- Long COVID: fatigue as commonest symptom; POTS as dysautonomia; WHO 3-month definition.
- "No-benefit" drugs: hydroxychloroquine, ivermectin, lopinavir-ritonavir, convalescent plasma.
- Remdesivir target: RNA-dependent RNA polymerase; Paxlovid (nirmatrelvir-ritonavir) target: main protease (Mpro/3CLpro).
- MIS-C in children: Kawasaki-like, 2–6 weeks post-infection.
Rapid revision
- SARS-CoV-2 = enveloped positive-sense ssRNA betacoronavirus; enters via ACE2 + TMPRSS2.
- Spike protein = vaccine/monoclonal target; N protein = rapid antigen test target.
- Anosmia/ageusia are highly specific early features.
- Investigation of choice = RT-PCR; rapid antigen has low sensitivity (positive confirms, negative doesn't exclude).
- CT: peripheral, bilateral, subpleural ground-glass opacities + crazy-paving.
- Severity: SpO₂ < 90% or RR > 30 = severe; ARDS/shock/ventilation = critical.
- Bad-prognosis labs: lymphopenia, ↑D-dimer, ↑ferritin, ↑LDH, ↑CRP, ↑IL-6.
- Dexamethasone (RECOVERY) helps only oxygen-requiring patients; harmful without hypoxia.
- Remdesivir (RdRp inhibitor) — early/moderate disease; tocilizumab/baricitinib for hyperinflammation.
- All admitted patients get LMWH thromboprophylaxis; COVID is intensely prothrombotic.
- Mucormycosis = diabetes + steroids + hypoxia → liposomal amphotericin B + debridement.
- Long COVID (WHO ≥ 3 months): commonest symptom = fatigue; classic dysautonomia = POTS; "brain fog" is the cognitive hallmark.