- COVID-19 is an infectious disease caused by the novel coronavirus SARS-CoV-2.
- SARS-CoV-2 originates from the same family of viruses as the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and the Middle East Respiratory Syndrome Coronavirus (MERS-CoV), but it is not the same virus.
- This new virus, now named SARS-CoV-2, had not been previously identified in humans before it was first identified in Wuhan, China, in December 2019.
- The COVID-19 outbreak was declared a Public Health Emergency of International Concern (PHEIC) by the World Health Organisation on 30 January 2020 and a global pandemic on 11 March 2020.
- COVID-19 can cause a wide range of illnesses, from mild respiratory symptoms to severe complications and death.
- While the SARS-CoV-2 virus originated in animals and the initial human cases were "zoonotic", the virus has been transmitting from person to person since December 2019.
- There is increasing information on how SARS-CoV-2 spreads from one person to another. Most transmission seems to occur after prolonged close contact with an infected case. The possible transmission modes of the virus at present are thought to be similar to other respiratory viruses:
- Direct contact
- Indirect contact via contaminated surfaces
- Possibly through the faecal-oral route, because coronaviruses can also infect the gastrointestinal tract
- At present, we are also uncertain if there is one transmission mode that is more important than the others, and this is an important scientific question that we are trying to answer.
- Usually, infected persons are most contagious around the time their symptoms appear. Asymptomatic or pre-symptomatic transmissions have been documented for SARS-CoV-2, where infected persons can be infectious even without showing any symptoms or before their symptoms appear.
- COVID-19 has a broad clinical spectrum. Some infected persons show no symptoms; others may experience common symptoms of mild respiratory illness (e.g., fever, cough, tiredness, loss of taste or smell) to severe illness. 
- In the early stages of the COVID-19 pandemic, many infected persons, especially those with underlying medical conditions, developed complications and life-threatening illnesses. A small fraction of these illnesses were fatal.
- Common symptoms of COVID-19 include fever, malaise and respiratory symptoms such as dry cough and shortness of breath. Other symptoms include aches and pains, nasal congestion, headache, conjunctivitis, sore throat, diarrhoea, or loss of taste or smell. Some identified cases have pneumonia. 
- Symptomatic cases usually show clinical signs and illness 1–14 days after they have been infected, most commonly around 5 days.
- At present, older adults and adults with underlying health conditions appear to be at the highest risk of severe disease.
Several vaccines for COVID-19 prevention have recently been rolled-out for use under different conditions (e.g., early use, emergency use, limited use) in a number of countries. Dozens of other candidate vaccines are in different stages of development, from pre-clinical investigation to large-scale trials. The health system in most jurisdictions adopts a priority-based allocation framework for the COVID-19 vaccine, enabling populations with the highest risks to be vaccinated first, such as healthcare workers, the elderly and those with pre-existing diseases.
For populations who have not yet received the vaccine, preventive measures are mostly non-pharmaceutical:
Personal protective measures
- Maintain good hand hygiene
- Practice good respiratory etiquette
- Wear a surgical mask
- Avoid touching eyes, nose and mouth
- Disinfect frequently touched objects and surfaces
- Inspect drainage pipes regularly and make sure the drain outlets contain water by adding half a litre of water into each drainage trap (U-trap) about once a week
Social distancing measures
- Isolation of infected persons at home or in hospitals
- Quarantine of potentially infected persons (e.g. those who have been in prolonged close contact with known cases) either at home or in special quarantine facilities
- Refrain from visiting crowded places
- Maintain (at least 3 feet) distance from other persons, especially those who are showing respiratory illness symptoms
- Workplaces can consider measures such as working from home or working in staggered shifts
- The World Health Organisation recommendations on therapeutics and COVID-19 are continuously updated with treatment options that  include the use of remdesivir, anti-inflammatory drugs, antivirals such as Paxlovid (nirmatrelvir-ritonavir), Molnupiravir, and other therapeutic options such as convalescent plasma.
- Researchers are trying to better understand the transmission dynamics and severity of COVID-19 from the documentation of outbreaks and chains of transmission.
- This information would affect how control and mitigation measures should be implemented; for example, the incubation period and viral shedding of infected persons would provide insights on the duration of quarantine and isolation, respectively.
- Large-scale epidemiological studies are also being conducted to understand the long-term risks and impact of COVID-19 in the community. Please visit https://covid19.med.hku.hk/en/cvc/studies for more information about the studies conducted by HKUMed.
- The latest research conducted by HKU SPH on COVID-19 is available at https://covid19.med.hku.hk/en/Research-on-COVID-19.
- Organization WH. Coronavirus disease (COVID-19) World Health Organization: World Health Organization; 2023 [cited 2023 2023Mar22]. Available from: https://www.who.int/health-topics/coronavirus#tab=tab_3.
- Looi M-K. How are covid-19 symptoms changing? BMJ. 2023;380:p3.
- World Health Organization. Therapeutics and COVID-19: living guideline. 2023. [cited: 2023 April 18]. Available from: https://www.who.int/publications/i/item/WHO-2019-nCoV-therapeutics-2023.1.
- Liu J, Pan X, Zhang S, Li M, Ma K, Fan C, et al. Efficacy and safety of Paxlovid in severe adult patients with SARS-Cov-2 infection: a multicenter randomized controlled study. Lancet Reg Health West Pac. 2023:100694.
Last updated 1 June 2023