Clinical features and sequelae
The most common symptoms of COVID-19 are:
- Loss of smell;
- Nasal obstruction;
- Rhinorrhoea; and
- Sore throat [5-9].
Symptoms may vary, both in frequency and severity, depending on the SARS-CoV-2 variant causing the disease episode . Most cases of COVID-19 are mild or moderate and do not require hospitalisation or advanced medical care .
Severe disease usually manifests as pneumonia with shortness of breath and pulmonary infiltrates on chest imaging. Pneumonia can be complicated by respiratory failure requiring oxygen supplementation and mechanical ventilation . Other severe complications include thromboembolism (such as pulmonary embolism and stroke), circulatory shock, myocardial damage, arrhythmias, and encephalopathy [11-13]. Severe illness usually develops approximately one week after the onset of symptoms.
People who received a full course of vaccination may still experience flu-like symptoms but are less likely to suffer from severe disease and require hospitalisation.
COVID-19 in children
Children usually experience mild symptoms (mainly fever and cough), if any, and have a very low risk of hospitalisation or death  . However, some children may develop severe disease after infection with COVID-19, defined as multi-system inflammatory syndrome in children (MIS-C). This rare syndrome shares common clinical features with other paediatric inflammatory syndromes such as Kawasaki disease, toxic shock syndrome, and macrophage activation syndrome [15,16].
Risk groups for severe disease
The risk of very severe disease increases with advancing age and the presence of comorbidities, and is slightly higher among males than females.
A range of conditions have been linked to severe clinical outcomes in terms of hospital admission, ICU admission, and mortality. These include hypertension, diabetes, chronic kidney disease, coronary heart disease, chronic obstructive pulmonary disease (COPD), cerebrovascular disease and chronic liver disease, use of immunosuppressive medications, arrythmia, ischemic heart disease, heart failure, cancer, and obesity [17-19]. Smoking has also been linked with a higher risk of severe outcomes .
Some migrant and ethnic minority groups have been found to be at an increased risk of severe illness and hospitalisation due to COVID-19. This is likely associated with social and economic factors, such as occupation (with face-to-face exposure), housing situation (with crowded and/or intergenerational housing), and barriers in access to healthcare .
Some patients may experience long-term symptoms with unclear aetiology, referred to as Post-COVID-19 condition (and sometimes as ‘Long COVID’). This has been defined as ‘the condition that occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually three months from the onset of COVID-19, with symptoms that last for at least two months and cannot be explained by an alternative diagnosis’.
The presentation is often episodic and affects multiple organ systems. This could include:
- Respiratory manifestations, e.g. shortness of breath, cough, and sore throat. Decreased diffusing capacity and abnormalities in lung imaging are commonly observed .
- Neuropsychiatric and cognitive symptoms, such as chronic fatigue (most commonly), headaches and loss of smell, difficulty concentrating, sleep disturbances, and depression .
- Cardiovascular presentations, e.g. chest pain and arrhythmias (e.g. atrial fibrillation presenting as palpitations or tachycardia), as well as heart failure and thromboembolic events .
- Manifestations from other organ systems, such as endocrine, gastrointestinal, renal, and skin, are reported less frequently .
Patients with co-morbidities, obese individuals, older adults (>50 years and, particularly, those >85 years), women, and hospitalised patients are more likely to report prolonged symptoms  .
Post-COVID-19 condition has been also reported in cohorts of children from several countries .