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A pandemic (/pænˈdɛmɪk/ pan-DEM-ik) is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic disease with a stable number of infected individuals is not a pandemic. Widespread endemic diseases with a stable number of infected individuals such as recurrences of seasonal influenza are generally excluded as they occur simultaneously in large regions of the globe rather than being spread worldwide.


In 2009, the World Health Organization (WHO) dropped the words "with enormous numbers of deaths and illness" from their definition.[15] In 2008, it also dropped the requirement of an "influenza pandemic" to be a new sub-type with a simple reassortant virus, meaning that many seasonal flu viruses now could be classified as pandemic influenza.[16]

The World Health Organization previously applied a six-stage classification to describe the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. It starts when mostly animals are infected with a virus and a few cases where animals infect people, then moves to the stage where the virus begins to be transmitted directly between people and ends with the stage when infections in humans from the virus have spread worldwide. In February 2020, a WHO spokesperson clarified that "there is no official category [for a pandemic]".[a][17]

During the 2009 influenza pandemic, Dr. Keiji Fukuda, Assistant Director-General ad interim for Health Security and Environment, WHO said "An easy way to think about pandemic ... is to say: a pandemic is a global outbreak. Then you might ask yourself: 'What is a global outbreak?' Global outbreak means that we see both the spread of the agent ... and then we see disease activities in addition to the spread of the virus."[19]

In planning for a possible influenza pandemic, the WHO published a document on pandemic preparedness guidance in 1999, which was revised in 2005 and 2009. It defined phases and appropriate actions for each phase in an aide-mémoire titled WHO pandemic phase descriptions and main actions by phase. The 2009 revision, including descriptions of a pandemic and the phases leading to its declaration, was finalized in February 2009. The 2009 H1N1 virus pandemic was neither on the horizon at that time nor mentioned in the document.[20][21] All versions of this document refer to influenza. The phases are defined by the spread of the disease; virulence and mortality are not mentioned in the current WHO definition, although these factors have previously been included.[22]

In 2014, the United States Centers for Disease Control and Prevention introduced an analogous framework to the WHO's pandemic stages titled the Pandemic Intervals Framework.[23] It includes two pre-pandemic intervals,

In 2014, the United States Centers for Disease Control and Prevention adopted the Pandemic Severity Assessment Framework (PSAF) to assess the severity of pandemics.[23] The PSAF superseded the 2007 linear Pandemic Severity Index, which assumed 30% spread and measured case fatality rate (CFR) to assess the severity and evolution of the pandemic.[25]

Historically, measures of pandemic severity were based on the case fatality rate.[26] However, the case fatality rate might not be an adequate measure of pandemic severity during a pandemic response because:[24]

To account for the limitations of measuring the case fatality rate alone, the PSAF rates the severity of a disease outbreak on two dimensions: clinical severity of illness in infected persons; and the transmissibility of the infection in the population.[24] Each dimension can be measured using more than one metric, which are scaled to allow comparison of the different metrics. Clinical severity can instead be measured, for example, as the ratio of deaths to hospitalizations or using genetic markers of virulence. Transmissibility can be measured, for example, as the basic reproduction number R0 and serial interval or via underlying population immunity. The framework gives guidelines for scaling the various measures and examples of assessing past pandemics using the framework.

A key part of managing an infectious disease outbreak is trying to decrease the epidemic peak, known as "flattening the curve".[28][31] This helps decrease the risk of health services being overwhelmed and provides more time for a vaccine and treatment to be developed.[28][31] A broad group of the so-called non-pharmaceutical interventions may be taken to manage the outbreak.[31] In a flu pandemic, these actions may include personal preventive measures such as hand hygiene, wearing face-masks, and self-quarantine; community measures aimed at social distancing such as closing schools and canceling mass gatherings; community engagement to encourage acceptance and participation in such interventions; and environmental measures such as cleaning of surfaces.[29]

Another strategy, suppression, requires more extreme long-term non-pharmaceutical interventions to reverse the pandemic by reducing the basic reproduction number to less than 1. The suppression strategy, which includes stringent population-wide social distancing, home isolation of cases, and household quarantine, was undertaken by China during the COVID-19 pandemic where entire cities were placed under lockdown; such a strategy may carry with it considerable social and economic costs.[37]

Although the WHO uses the term "global epidemic" to describe HIV (.mw-parser-output cite.citationfont-style:inherit; .citation qquotes:"\"""\"""'""'".mw-parser-output .citation:targetbackground-color:rgba(0,127,255,0.133).mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free abackground:url("//")right 0.1em center/9px .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration abackground:url("//")right 0.1em center/9px .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription abackground:url("//")right 0.1em center/9px .cs1-ws-icon abackground:url("//")right 0.1em center/12px .cs1-codecolor:inherit;background:inherit;border:none; .cs1-hidden-errordisplay:none; .cs1-maintdisplay:none;color:#3a3; .citation .mw-selflinkfont-weight:inherit"WHO HIV/AIDS Data and Statistics". Retrieved 12 April 2020.), as HIV is no longer an uncontrollable outbreak outside of Africa, some authors use the term "pandemic".[38]HIV originated in Africa, and spread to the United States via Haiti between 1966 and 1972.[39] AIDS is currently a pandemic in Africa, with infection rates as high as 25% in some regions of southern and eastern Africa. In 2006, the HIV prevalence among pregnant women in South Africa was 29%.[40] Effective education about safer sexual practices and bloodborne infection precautions training have helped to slow down infection rates in several African countries sponsoring national education programs.[citation needed] There were an estimated 1.5 million new infections of HIV/AIDS in 2020. As of 2020[update] there have been about a total of 32.7 million deaths related to HIV/AIDS since the epidemic started.[41]

SARS-CoV-2, a new strain of coronavirus, was first detected in the city of Wuhan, Hubei Province, China, in December 2019.[43] It caused a pandemic of cases of an acute respiratory disease, which is referred to as coronavirus disease 2019 (COVID-19).[31] More than 200 countries and territories have been affected by COVID-19, with major outbreaks occurring in Brazil, Russia, India, Mexico, Peru, South Africa,[44][45] Western Europe, and the United States.[31] On 11 March 2020, the World Health Organization characterized the spread of COVID-19 as a pandemic, marking the first global pandemic since the 2009 swine flu pandemic.[31][46][47] As of 31 January 2023, the number of people infected with COVID-19 has reached more than 670 million worldwide. The current death toll is 6,831,146.[48] It is believed that these figures underestimate true totals as testing did not commence in the initial stages of the outbreak and many people infected by the virus have no or only mild symptoms so may not have been tested. Similarly, the number of deaths may also be understated as fatalities may have not been tested or are attributed to other conditions.[31] This was especially the case in large urban areas where a non-trivial number of patients died while in their private residences.[31]

In a press conference on 28 December 2020, Dr. Mike Ryan, head of the WHO Emergencies Program, and other officials said the current COVID-19 pandemic is "not necessarily the big one" and "the next pandemic may be more severe." They called for preparation.[179] The WHO and the UN, have warned the world must tackle the cause of pandemics and not just the health and economic symptoms.[180]

In June 2021, a team of scientists assembled by the Harvard Medical School Center for Health and the Global Environment warned that the primary cause of pandemics so far, the anthropogenic destruction of the natural world through such activities including deforestation and hunting, is being ignored by world leaders.[184] 041b061a72


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