Molecular structure pattern

Coronavirus Disease 2019 (COVID-19) Disease Background

Last updated: 25 September 2026
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Introduction

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

Epidemiology

Globally, as of 30 August 2026, there have been 779,366,377 total confirmed cases of COVID-19 and 7,116,554 deaths according to the World Health Organization (WHO). 

Per region, confirmed cases based on WHO data as of 30 August 2026 are as follows: 

  • Americas: 193,917,141
  • Europe: 281,759,177
  • Southeast Asia: 55,101,182
  • Eastern Mediterranean: 23,417,911
  • Africa: 9,590,195
  • Western Pacific: 215,441,060      

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. Through mutation and recombination, the virus evolved from its original strain into variants of concern (VOC) such as Alpha, Beta, Gamma, Delta, and Omicron. 

A VOC 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. The last designated VOC was Omicron in 2021. Since 2022, Omicron and its sublineages have predominated globally, sharing features such as high transmissibility driven by immune escape and generally lower intrinsic disease severity.

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. The last circulating variant of interest was identified in 2024, which was JN.1 (excluding JN.1 sublineages).

Variants under monitoring (VUMs), based on WHO 2023 working definition, 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 as 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 27 July 2026
Pango Lineage  Nexstrain Clade  Genetic Features  Earliest Documented Samples
PQ.16.1.1  25B  NB.1.8.1 + S:D253G, S:N417T, S:D420N, S:I478T  25 March 2024 
NB.1.8.1  25B  JN1 + S:T22N, S:F59S, S:G184S, S:A435S, S:F456L, S:T478I, S:Q493E  22 January 2025 
XFG  25C  JN1 + S:T22N, S;S31P, S:K182R, S:R190S, S:R346T, S:K444R, S:V445R, S:F456L, S:N487D, S:Q493E, S:T572I  27 January 2025 
BA.3.2     JN1 P9L, R21T, P26L, A67V, H69-, V70-, T95I, I101T, C136-, N137-, D138-, P139-, F140-, L141, G142-, V143-, Y144-, Y145-, H146-, K147-, F157S, N164K, S172F, K187T, N211-, L212I, A243, L244-, P251S, I326V, G339Y, A348P, S371F, S373P, S375F, R403K, D405N, R408S, K417N, A435S, N440R, V445A, G446D, L452W, N460K, S477N, T478N, E484K, G496S, Q498R, N501Y , K529N, E554D, E583D, D614G, H625R, N641K, V642G, E654K, H655Y , N679R, P681R, A688D, S704L, N764K, K795T, D796Y, A852K, S939F, Q954H, N969K, P1162R, D1184E  22 November 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 primarily through infectious particles transmitted at close range and in poorly ventilated indoor environments. Contact and droplet transmission can also happen 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 may occur as 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. SARS-CoV-2 has also been detected in other biological samples (eg urine, feces, plasma or serum), but transmission has not been documented.

Incubation Period 

The incubation period ranges from 2-14 days (2-4 days, for the Omicron variant).

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