COVID-19 — Causes, Symptoms, Variants & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first identified in Wuhan, China in December 2019. It has since caused a global pandemic with over 700 million confirmed infections and at least 7 million reported deaths — with true excess mortality estimated at 15-20 million. SARS-CoV-2 is an enveloped positive-sense single-stranded RNA betacoronavirus that binds to ACE2 receptors on human cells via its spike protein. The virus has undergone significant evolution producing successive variants of concern (VOC): Alpha, Beta, Delta, and Omicron (with multiple sub-lineages — BA.4/5, BQ.1, XBB.1.5, JN.1, KP.2). As of 2024-2026, COVID-19 is endemic globally, with seasonal waves driven by waning immunity and antigenic evolution. Omicron sub-lineages cause predominantly upper respiratory illness in vaccinated individuals; severe disease and death disproportionately affect the elderly, immunocompromised, and those with significant comorbidities.

Transmission: primarily respiratory — close contact exposure to respiratory droplets and aerosols from infected individuals (singing, coughing, speaking in poorly ventilated spaces). Fomite transmission is possible but less significant. Incubation period: 2-14 days (Omicron: 2-5 days, shorter than earlier variants). High-risk settings: crowded indoor poorly-ventilated environments, healthcare settings without adequate PPE. Pathogenesis: SARS-CoV-2 enters cells via ACE2 receptor (abundant in lung type II pneumocytes, cardiovascular endothelium, intestinal epithelium, and nasal epithelium). Severe disease involves dysregulated immune response with cytokine storm, endothelial inflammation, hypercoagulability (micro- and macro-thrombosis — causing stroke, PE, DVT, MI), and acute respiratory distress syndrome (ARDS). Risk factors for severe disease: age over 65 (most significant), obesity (BMI over 35), diabetes mellitus, chronic cardiorespiratory disease (COPD, heart failure, asthma), chronic kidney disease, immunocompromise (haematological malignancy, organ transplant, systemic immunosuppressive therapy), and unvaccinated status.

Mild-moderate disease (80-85%): fever, cough, fatigue, sore throat, rhinorrhoea, headache, muscle aches, and loss of taste or smell (anosmia/ageusia — more common with early variants; less so with Omicron). Moderate disease (10-15%): persistent fever, dyspnoea (breathlessness on moderate exertion), and oxygen saturation decline (SpO2 95-96%). Severe disease (5%): significant dyspnoea at rest (SpO2 below 94% on air), rapid respiratory rate (above 30/min), hypoxaemia requiring supplemental oxygen, and markers of systemic inflammation. Critical disease (less than 1-2% in vaccinated): respiratory failure requiring mechanical ventilation, multi-organ failure, septic shock. COVID-19 symptoms with Omicron sub-lineages more closely resemble influenza or upper respiratory infection in vaccinated people. Post-COVID-19 condition (Long COVID): persistent symptoms lasting over 12 weeks after acute infection — fatigue, cognitive impairment ('brain fog'), breathlessness, chest pain, post-exertional malaise. Affects an estimated 10-20% of infected individuals; more common in women aged 35-69.

Rapid antigen tests (lateral flow tests/LFT): detect SARS-CoV-2 nucleocapsid antigen; highly specific (specificity 97-99%) but lower sensitivity than PCR, particularly early in infection or with Omicron sub-lineages. Best performed on day 1-3 of symptoms for maximum sensitivity. Widely available, results within 15-30 minutes. PCR (RT-PCR): most sensitive test (gold standard), detecting viral RNA; remains positive for 7-10 days in mild-moderate disease. Used in healthcare settings, immunocompromised patients, and for clinical confirmation when LFT is negative but symptoms are typical. Clinical assessment: in hospitalised patients, chest X-ray (bilateral ground-glass opacities in COVID-19 pneumonia), CT thorax (more sensitive for pneumonitis extent), ABG/SpO2 monitoring, blood tests (FBC, CRP, ferritin, D-dimer, LDH, LFTs, renal function, troponin — elevated in cardiac involvement). WHO definition of severe COVID-19: SpO2 below 90% on room air, respiratory rate above 30/min, or signs of severe respiratory distress. Critical COVID-19: life-threatening respiratory failure, septic shock, or multi-organ dysfunction.

Mild-moderate disease (ambulatory): most cases require only symptom management — paracetamol/ibuprofen for fever and pain, adequate hydration, and rest. Antivirals for high-risk patients with mild-moderate disease: nirmatrelvir/ritonavir (Paxlovid) — 5-day course started within 5 days of symptom onset reduces hospitalisation and death by 89% in unvaccinated high-risk patients (less benefit in vaccinated population but still recommended for highest-risk); remdesivir (Veklury) IV — 3-day infusion for outpatients at high risk within 7 days of symptom onset. Molnupiravir: less effective alternative when other antivirals are contraindicated. Hospitalised patients requiring supplemental oxygen: dexamethasone 6 mg IV/oral for 10 days reduces mortality by 20-30% (RECOVERY trial — landmark result); remdesivir reduces time to recovery. Severe/critical disease: high-flow nasal oxygen, prone positioning (improves oxygenation), non-invasive ventilation, and mechanical ventilation in ICU. Baricitinib (JAK inhibitor) reduces mortality in critically ill patients. Anticoagulation: prophylactic LMWH for all hospitalised COVID-19 patients (thromboembolic risk). COVID-19 vaccines (mRNA — Pfizer/BioNTech Comirnaty, Moderna Spikevax; protein subunit — Novavax Nuvaxovid) remain the most effective public health intervention, substantially reducing severe disease, hospitalisation, and death.

COVID-19 causes a spectrum of complications affecting multiple organ systems, with risk proportional to age, comorbidities, and vaccination status. Respiratory complications are the most common: severe pneumonitis progressing to acute respiratory distress syndrome (ARDS) requiring mechanical ventilation occurs in approximately 5% of hospitalised patients; post-COVID lung fibrosis causes persistent breathlessness and reduced exercise capacity in a proportion of survivors. Thromboembolic complications are a hallmark of severe COVID-19 — hypercoagulability from endothelial inflammation causes deep vein thrombosis, pulmonary embolism, stroke, and myocardial infarction even in previously healthy individuals; prophylactic anticoagulation is standard for all hospitalised patients. Cardiac complications include myocarditis and pericarditis (more common in young males, associated with mRNA vaccination at very low frequency), new-onset arrhythmias, and acute heart failure. Multisystem inflammatory syndrome in children (MIS-C) — a rare but serious post-COVID immune response — causes fever, rash, gastrointestinal symptoms, and cardiac inflammation requiring urgent immunomodulatory treatment. Secondary bacterial and fungal infections (particularly invasive pulmonary aspergillosis in ICU patients receiving corticosteroids) complicate severe COVID-19. Post-COVID-19 condition (Long COVID): persistent symptoms beyond 12 weeks — fatigue, post-exertional malaise, cognitive impairment, breathlessness, and palpitations — affects an estimated 10–20% of infected individuals and represents a significant long-term public health burden.

Vaccination is the primary prevention strategy. Primary vaccination series with updated COVID-19 vaccines provides significant protection against severe disease, hospitalisation, and death. Annual COVID-19 boosters are recommended for older adults (aged 65+), immunocompromised individuals, care home residents, and healthcare workers — aligned with dominant circulating variants where possible. Non-pharmacological measures remain relevant for high-risk individuals and in healthcare settings: well-fitting FFP2/N95 respirators provide superior protection to surgical masks; adequate indoor ventilation (CO2 monitoring, HEPA air filtration); and avoiding crowded poorly-ventilated indoor spaces during high-transmission periods. Early antiviral treatment for high-risk individuals minimises progression. Hydroxychloroquine, ivermectin, and other widely promoted remedies have no proven efficacy in COVID-19.

Call emergency services (999/911) immediately for: severe breathlessness at rest; SpO2 below 94% on pulse oximeter; coughing or vomiting blood; chest pain that is persistent or severe; confusion, difficulty waking, or loss of consciousness; blue-tinged lips or face (cyanosis); or severe abdominal pain with fever. High-risk individuals (over 65, immunocompromised, significant comorbidities) should seek medical advice within 5 days of symptom onset to assess eligibility for antiviral therapy (Paxlovid/remdesivir). Contact a GP or healthcare provider for persistent fever above 39°C for more than 3 days, breathlessness that worsens over 5-7 days, or worsening symptoms after initial mild course. Most healthy vaccinated adults with COVID-19 can safely recover at home.