Coronavirus Disease 2019 (COVID-19) Disease Background

Last updated: 06 May 2025

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Introduction

Content on this page:

Introduction

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 30 March 2025, there have been 776,691,501 total confirmed cases of COVID-19 according to the World Health Organization (WHO). There have been 7,093,267 confirmed deaths as of 30 March 2025. Per region, confirmed cases based on WHO data as of 30 March 2025 are as follows: 

  • Americas: 193,520,137
  • Europe: 281,221,455
  • Southeast Asia: 61,332,810
  • Eastern Mediterranean: 23,417,911
  • Africa: 9,587,638
  • Western Pacific: Western Pacific: 208,610,786   

In South-East Asia, India has the most number of cases being affected by the disease. This is followed by Indonesia, Thailand and Bangladesh. In the whole region, as of 11 February 2024, 808,452 lives have been confirmed to be lost due to COVID-19.

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 



As of 28 March 2025, there is no VOC.

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.

 Circulating Variant of Interest (VOI) as of 02 December 2024
 Pango Lineage  Nexstrain Clade  Genetic Features  Earliest Documented Samples
JN.1
(Excludes JN.1 sublineages listed as VUMs)		 24A 
 BA.2.86 + S:L455S 
 25 August2023

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). 

Variants under monitoring (VUMs), based on WHO working definition by 4 October 2023, are variants of SARS-CoV-2 with genetic changes that are suspected to affect the characteristics of the virus and the early signals of growth advantage relative to other circulating variants (eg growth advantage which can occur globally or in only one WHO region), but with unclear evidence of phenotypic or epidemiological impact, requiring enhanced monitoring and reassessment pending new evidence. It is designated VUM if the variant has an unusually large number of mutations in known antigenic sites, but with very few sequences and not possible to estimate its relative growth advantage and if there is also evidence of community transmission in ≥2 countries within a 2- to 4-week period.      
 Circulating Variants Under Monitoring (VUMs) as of 14 April 2025
Pango Lineage  Nexstrain Clade  Genetic Features  Earliest Documented Samples
KP.3
 24C JN.1 + S:F456L, S:Q493E, S:V1104L
 11 February 2024
KP.3.1.1
 24C K.P.3 + S:S31-
 27 March 2024
LB.1 24A JN.1 + S:S31-, S:Q183H, S:R346T, S:F456L 26 April 2024
LP.8.1  24B  JN1 + S:S31-, S:F186L, S:R190S, S:R346T, S:V445R, S:F456L, S:Q493E, S:K1086R, S:V1104L  01 July 2024 
XEC 24F JN.1 + S:T22N, S:F59S, S:F456L, S:Q493E, S:V1104L 16 June 2024

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).


Coronavirus Disease 2019 (COVID-19)_Disease BackgroundCoronavirus Disease 2019 (COVID-19)_Disease Background



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