COVID-19

This is a respiratory illness caused by infection with severe acute respiratory syndrome coronavirus 2. It ranges from asymptomatic or mild upper respiratory infection to severe pneumonia with respiratory failure and multiorgan involvement.

The clinical landscape has evolved since the initial pandemic phase. Vaccination, accumulated population immunity, and specific antiviral and immunomodulatory treatments have changed outcomes.

Pathophysiology

The virus binds to the angiotensin converting enzyme 2 receptor, expressed on respiratory epithelium and other tissues. It enters cells and triggers a direct viral cytopathic effect.

In more severe disease, a dysregulated host immune response produces excessive inflammatory cytokines. This contributes to the characteristic pattern of progressive respiratory deterioration seen in a subset of patients, typically in the second week of illness.

A recognised prothrombotic tendency contributes to thromboembolic complications.

The clinical spectrum is broad:

• Asymptomatic infection.
• Mild upper respiratory symptoms (sore throat, nasal congestion, cough, fatigue).
• More significant illness with fever, myalgia and, characteristically though not universally, altered or lost sense of taste and smell.
• Severe pneumonia with progressive breathlessness and hypoxia.
• In the most severe cases, acute respiratory distress syndrome requiring intensive respiratory support.

A biphasic pattern is characteristic in more severe disease. Initial mild symptoms are followed by clinical deterioration, often around the end of the first week of illness. This reflects the transition from a predominantly viral replicative phase to an inflammatory phase driving the more severe pulmonary manifestations.

Risk factors for severe disease include older age, obesity, diabetes, cardiovascular disease, chronic respiratory disease, immunocompromise and pregnancy. Vaccination status significantly modifies the risk of severe outcomes.

Post-acute sequelae (long COVID) comprise a range of persistent symptoms, including fatigue, breathlessness, cognitive difficulty and other multisystem manifestations. They persist beyond the acute illness in a proportion of patients regardless of initial disease severity. They remain a significant and still incompletely understood part of the overall disease burden.

Polymerase chain reaction testing or rapid antigen testing on a nasal or nasopharyngeal swab confirms the diagnosis, with polymerase chain reaction offering superior sensitivity, particularly relevant in settings where confirmation would guide treatment or infection control decisions.

Chest imaging (chest radiograph, or computed tomography for more detailed assessment) is reserved for patients with significant respiratory symptoms or hypoxia, showing characteristic bilateral peripheral ground glass changes in more severe pulmonary disease, rather than used routinely for confirmation of diagnosis in mild presentations.

Blood tests (full blood count, inflammatory markers, renal and liver function) and pulse oximetry support severity assessment in patients with more significant illness, with specific markers (D-dimer, given the prothrombotic tendency) considered where thromboembolic complications are suspected.

Supportive care forms the foundation of management for the majority of presentations managed in the community. It includes adequate hydration, rest, analgesia and antipyretics, and monitoring of oxygen saturation.

Specific antiviral treatment (oral agents targeting viral replication) is available for patients at high risk of progression to severe disease. It is most effective when started early in the course of illness. Eligibility is guided by risk factor assessment, reflecting the reduced risk of severe outcomes in the broader, increasingly immune population.

Supplemental oxygen is provided to maintain adequate saturation in hospitalised patients. Progressive respiratory failure is escalated to non invasive or invasive ventilatory support according to standard respiratory support escalation pathways.

Corticosteroids (dexamethasone) have established evidence of mortality benefit in patients requiring supplemental oxygen, reflecting the contribution of dysregulated inflammation to severe disease. Additional immunomodulatory therapy (targeting specific inflammatory pathways) is considered for selected patients with progressive severe disease under specialist guidance.

Thromboprophylaxis is considered for hospitalised patients given the recognised prothrombotic tendency associated with this infection. Therapeutic anticoagulation is reserved for confirmed thromboembolic complications or specific high risk clinical scenarios.

Vaccination remains a central preventive strategy, reducing the risk of severe disease, hospitalisation and death. Vaccination schedules and booster recommendations evolve according to ongoing evidence and population risk stratification.

Management of persistent post-acute symptoms (long COVID) is structured, symptom directed and multidisciplinary. It includes graded activity advice tailored to avoid symptom exacerbation, and specific management of individual persistent symptoms (breathlessness, cognitive difficulty, fatigue) according to their nature and severity. It is generally delivered through dedicated post-COVID assessment and rehabilitation services where available.