Coronavirus Disease 2019 (COVID-19) Disease Background

Introduction 

Coronavirus disease 2019 (COVID-19) is a respiratory tract infection caused by a newly emergent coronavirus, SARS-CoV-2, first reported in Wuhan, China in December 2019.

Epidemiology 

Globally as of 11 February 2024, there have been 774,631,444 confirmed cases of COVID-19, including 7,031,216 deaths, reported to the World Health Organization (WHO). Per region, confirmed cases based on WHO data as of 11 February 2024 are as follows: 

• Americas: 193,179,606
• Europe: 278,992,772
• Eastern Mediterranean: 23,412,773
• Africa: 9,575,413
• Western Pacific: 208,210,078 

In South-East Asia, as of 11 February 2024, there are a total of 61,260,088 confirmed cases of COVID-19. Among the countries in this region, India has the most number of cases with 45,028,732 people being affected by the disease. This is followed by Indonesia with 6,828,367 cases, Thailand with 4,766,751 cases, and Bangladesh with 2,048,149 cases. In the whole region, as of 11 February 2024, 808,452 lives have been confirmed to be lost due to COVID-19. In the Philippines, as of 11 December 2023, there have been a total of 4,127,856 confirmed cases with as much as 66,779 deaths.  

Etiology 

SARS-CoV-2 is classified within the genus Betacoronavirus (subgenus Sarbecovirus) of the family Coronaviridae. It is an enveloped, positive-sense, single-stranded ribonucleic acid (RNA) virus with a 30-kb genome. SARS-CoV-2 is most genetically similar to SARS-CoV-1 and both belong to the subgenus Sarbecovirus within the genus Betacoronavirus; however, SARS-CoV-1 is currently not known to circulate in the human population. 

A variant of concern is a SARS-CoV-2 variant with genetic changes that are predicted or known to affect virus characteristics such as transmissibility, virulence, antibody evasion, susceptibility to therapeutics, and detectability. It is also identified to have a growth advantage over other circulating variants in more than one WHO region with increasing relative prevalence alongside an increasing number of cases over time, or other apparent epidemiological impacts to suggest an emerging risk to global public health. As of 18 May 2023, there is no variant of concern. Variant of concern would also need to meet one of the following criteria: 

• Detrimental change in clinical disease severity
• Change in COVID-19 epidemiology causing a substantial impact on the ability of health systems to provide care to patients with COVID-19 or other illnesses thus requiring major public health interventions
• Significant decrease in the effectiveness of available vaccines in protecting against severe disease 

A variant of interest is a SARS-CoV-2 variant with genetic changes that are predicted or known to affect virus characteristics such as transmissibility, virulence, antibody evasion, susceptibility to therapeutics, and detectability. It is also identified to have a growth advantage over other circulating variants in more than one WHO region with an increasing relative prevalence alongside an increasing number of cases over time, or other apparent epidemiological impacts to suggest an emerging risk to global public health.

As of 15 March 2023, WHO will assign Greek letters to variants of concern while variants of interest will be referred to using established scientific nomenclature systems (ie Nexstrain and Pango). 

Pathophysiology 

Infection is caused by binding of the viral surface spike protein to the human angiotensin-converting enzyme 2 (ACE2) receptor after activation of the spike protein by transmembrane protease serine 2.

Mode of Transmission

The mode of transmission of COVID-19 is by contact and droplet transmission through direct, indirect, or close contact with infected individuals through secretions (ie saliva and respiratory secretions). Airborne transmission occurs during medical procedures that generate aerosols (aerosol-generating procedures).

Fomite transmission is through contaminated surfaces and objects. Viable SARS-CoV-2 virus and/or RNA detected by reverse transcription-polymerase chain reaction (RT-PCR) can be found on those surfaces for periods ranging from hours to days, depending on the ambient environment (including temperature and humidity) and the type of surface.

Other modes of transmission include the urine, feces, plasma, or serum.

Incubation Period 

Generally, after exposure to the virus, the mean time to develop symptoms is 4 to 6 days, with a range of between 1 and 14 days. 

Risk Factors 

Factors that Determine Transmission Risk  

Transmission risk is determined by the following factors:

• Whether the virus is still replication-competent
• Presence of symptoms (eg cough)
• Behavior and environmental factors associated with the infected person
• COVID-19 patient starts to gradually produce neutralizing antibodies that reduce the risk of virus transmission, usually 5 to 10 days after infection with SARS-CoV-2