Pandemic

A pandemic ( ) 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. 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.

Throughout human history, there have been a number of pandemics of diseases such as smallpox. The most fatal pandemic in recorded history was the Black Death—also known as The Plague—which killed an estimated 75–200 million people in the 14th century. The term had not been used then but was used for later epidemics, including the 1918 influenza pandemic—more commonly known as the Spanish flu. The most recent pandemics include the HIV/AIDS pandemic, the 2009 H1N1 pandemic and the COVID-19 pandemic. Almost all these diseases still circulate consistently among humans though their impact now is far less.

In response to the COVID-19 pandemic, recently 194 member states of the World Health Organization began negotiations on an International Treaty on Pandemic Prevention, Preparedness and Response with a requirement to submit a draft of this treaty to the 77th World Health Assembly during its 2024 convention. This is expected to set rules for dealing with a pandemic by the international community.

Definition

A medical dictionary definition of pandemic is "''an epidemic occurring on a scale that crosses international boundaries, usually affecting people on a worldwide scale''". A disease or condition is not a pandemic merely because it is widespread or kills many people; it must also be infectious. For instance, cancer is responsible for many deaths but is not considered a pandemic because the disease is not contagious—i.e. easily transmissible—and not even simply infectious. This definition differs from colloquial usage in that it encompasses outbreaks of relatively mild diseases.

The World Health Organization's (WHO) nearest equivalent of "pandemic" is what is known as a Public Health Emergency of International Concern, defined as "''an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response''". There is a rigorous process underlying this categorization and a clearly defined trajectory of responses.

A WHO-sponsored international body, tasked with preparing an international agreement on pandemic prevention, preparedness and response has defined a pandemic as "''the global spread of a pathogen or variant that infects human populations with limited or no immunity through sustained and high transmissibility from person to person, overwhelming health systems with severe morbidity and high mortality, and causing social and economic disruptions, all of which require effective national and global collaboration and coordination for its control''".

The word comes from the Greek παν- ''pan-'' meaning "all", or "every" and δῆμος ''demos'' "people".

Assessment

Stages

For a novel influenza virus, the World Health Organization previously applied a six-stage classification to describe the process by which the 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 or a pandemic.

During the 2009 influenza pandemic, 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."

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. 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.

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. It includes two pre-pandemic intervals,
* Investigation
* Recognition
and four pandemic intervals,
* Initiation
* Acceleration
* Deceleration
* Preparation
It also includes a table defining the intervals and mapping them to the WHO pandemic stages.

Severity

In 2014, the United States Centers for Disease Control and Prevention adopted the Pandemic Severity Assessment Framework (PSAF) to assess the severity of pandemics. 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.

Historically, measures of pandemic severity were based on the case fatality rate. However, the case fatality rate might not be an adequate measure of pandemic severity during a pandemic response because:
* Deaths may lag several weeks behind cases, making the case fatality rate an underestimate
* The total number of cases may not be known, making the case fatality rate an overestimate
* A single case fatality rate for the entire population may obscure the effect on vulnerable sub-populations, such as children, the elderly, those with chronic conditions, and members of certain racial and ethnic minorities
* Fatalities alone may not account for the full effects of the pandemic, such as absenteeism or demand for healthcare services

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. 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.

Management

The basic strategies in the control of an outbreak are containment and mitigation. Containment may be undertaken in the early stages of the outbreak, including contact tracing and isolating infected individuals to stop the disease from spreading to the rest of the population, other public health interventions on infection control, and therapeutic countermeasures such as vaccinations which may be effective if available. When it becomes apparent that it is no longer possible to contain the spread of the disease, management will then move on to the mitigation stage, in which measures are taken to slow the spread of the disease and mitigate its effects on society and the healthcare system. In reality, containment and mitigation measures may be undertaken simultaneously.

A key part of managing an infectious disease outbreak is trying to decrease the epidemic peak, known as " flattening the curve". This helps decrease the risk of health services being overwhelmed and provides more time for a vaccine and treatment to be developed. A broad group of the so-called non-pharmaceutical interventions may be taken to manage the outbreak. 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.

Another strategy, suppression, requires more extreme long-term non-pharmaceutical interventions to reverse the pandemic by reducing the basic reproduction number to less than1. 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.

Notable pandemics and outbreaks

Recent outbreaks

COVID-19

SARS-CoV-2, a new strain of coronavirus, was first detected in the city of Wuhan, Hubei Province, China, in December 2019. The outbreak was characterized as a Public Health Emergency of International Concern (PHEIC) between January 2020 and May 2023 by the World Health Organization. The number of people infected with COVID-19 has reached more than 767 million worldwide, with a death toll of 6.9 million. It is considered likely that the virus will eventually become endemic and, like the common cold, cause less severe disease for most people.

HIV/AIDS

HIV/AIDS was first identified as a disease in 1981, and is an ongoing worldwide public health issue. Since then HIV/AIDS has killed an estimated 40 million people with a further 630,000 deaths annually; 39 million people are currently living with HIV infection. HIV has a zoonotic origin, having originated in nonhuman primates in Central Africa and transferred to humans in the early 20th century. The most frequent mode of transmission of HIV is through sexual contact with an infected person. There may be a short period of mild, nonspecific symptoms followed by an asymptomatic (but nevertheless infectious) stage called clinical latency - without treatment, this stage can last between 3 and 20 years. The only way to detect infection is by means of a HIV test. There is no vaccine to prevent HIV infection, but the disease can be held in check by means of antiretroviral therapy.

Pandemics in history

It is assumed that, prior to the neolithic revolution around 10,000 BC, disease outbreaks were limited to a single family or clan. and did not spread widely before dying out. The domestication of animals opened the possibility of zoonotic infections; the advent of agriculture, and trade between settled groups, made it possible for pathogens to spread widely. As population increased, contact between groups became more frequent. A history of epidemics maintained by the Chinese Empire from 243 B.C. to 1911 A.C. shows an approximate correlation between the frequency of epidemics and the growth of the population.

Here is an incomplete list of known epidemics which spread widely enough to merit the title "pandemic".
* Plague of Athens (430 to 426 BC): During the Peloponnesian War, typhoid fever killed a quarter of the Athenian troops and a quarter of the population. This disease fatally weakened the dominance of Athens, but the sheer virulence of the disease prevented its wider spread; i.e., it killed off its hosts at a rate faster than they could spread it. The exact cause of the plague was unknown for many years. In January 2006, researchers from the University of Athens analyzed teeth recovered from a mass grave underneath the city and confirmed the presence of bacteria responsible for typhoid.
* Antonine Plague (165 to 180 AD): Possibly measles or smallpox brought to the Italian peninsula by soldiers returning from the Near East, it killed a quarter of those infected, up to five million in total.
* Plague of Cyprian (251–266 AD): A second outbreak of what may have been the same disease as the Antonine Plague killed (it was said) 5,000 people a day in Rome.
* Plague of Justinian (541 to 549 AD): Also known as the ''FIrst Plague Pandemic''. This epidemic started in Egypt and reached Constantinople the following spring, killing (according to the Byzantine chronicler Procopius) 10,000 a day at its height, and perhaps 40% of the city's inhabitants. The plague went on to eliminate a quarter to half the human population of the known world and was identified in 2013 as being caused by bubonic plague.
* Black Death (1331 to 1353): Also known as the ''Second Plague Pandemic.'' The total number of deaths worldwide is estimated at 75 to 200 million. Starting in Asia, the disease reached the Mediterranean and western Europe in 1348 (possibly from Italian merchants fleeing fighting in Crimea) and killed an estimated 20 to 30 million Europeans in six years; a third of the total population, and up to a half in the worst-affected urban areas. It was the first of a cycle of European plague epidemics that continued until the 18th century; there were more than 100 plague epidemics in Europe during this period, including the Great Plague of London of 1665–66 which killed approximately 100,000 people, 20% of London's population.
* 1817–1824 cholera pandemic. Previously restricted to the Indian subcontinent, the pandemic began in Bengal, then spread across India by 1820. 10,000 British troops and thousands of Indians died during this pandemic. It extended as far as China, Indonesia (where more than 100,000 people succumbed on the island of Java alone) and the Caspian Sea before receding. Deaths in the Indian subcontinent between 1817 and 1860 are estimated to have exceeded 15 million. Another 23 million died between 1865 and 1917. Russian deaths during a similar period exceeded 2million.
* Third plague pandemic (1855–1960): Starting in China, it is estimated to have caused over 12 million deaths in total, the majority of them in India. During this pandemic, the United States saw its first outbreak: the San Francisco plague of 1900–1904. The causative bacterium, '' Yersinia pestis'', was identified in 1894. The association with fleas, and in particular rat fleas in urban environments, led to effective control measures. The pandemic was considered to be over in 1959 when annual deaths due to plague dropped below 200. The disease is nevertheless present in the rat population worldwide and isolated human cases still occur.
* The 1918–1920 Spanish flu infected half a billion people around the world, including on remote Pacific islands and in the Arctic—killing 20 to 100 million. Most influenza outbreaks disproportionately kill the very young and the very old, but the 1918 pandemic had an unusually high mortality rate for young adults. It killed more people in 25 weeks than AIDS did in its first 25 years. Mass troop movements and close quarters during World WarI caused it to spread and mutate faster, and the susceptibility of soldiers to the flu may have been increased by stress, malnourishment and chemical attacks. Improved transportation systems made it easier for soldiers, sailors and civilian travelers to spread the disease.

Pandemics in indigenous populations

Beginning from the middle ages, encounters between European settlers and native populations in the rest of the world often introduced epidemics of extraordinary virulence. Settlers introduced novel diseases which were endemic in Europe, such as smallpox, measles, pertussis.and influenza, to which the indigenous peoples had no immunity. The Europeans infected with such diseases typically carried them in a dormant state, were actively infected but asymptomatic, or had only mild symptoms.

Historical accounts of epidemics are often vague or contradictory in describing how victims were affected. A rash accompanied by a fever might be smallpox, measles, scarlet fever, or varicella, and many epidemics overlapped with multiple infections striking the same population at once, therefore it is often impossible to know the exact causes of mortality (although ancient DNA studies can often determine the presence of certain microbes).

Smallpox was the most destructive disease that was brought by Europeans to the Native Americans, both in terms of morbidity and mortality. The first well-documented smallpox epidemic in the Americas began in Hispaniola in late 1518 and soon spread to Mexico. Estimates of mortality range from one-quarter to one-half of the population of central Mexico. It is estimated that over the 100 years after European arrival in 1492, the indigenous population of the Americas dropped from 60 million to only 6 million, due to a combination of disease, war, and famine. The majority these deaths are attributed to successive waves of introduced diseases such as smallpox, measles, and typhoid fever.

In Australia, smallpox was introduced by European settlers in 1789 devastating the Australian Aboriginal population, killing an estimated 50% of those infected with the disease during the first decades of colonisation. In the early 1800s, measles, smallpox and intertribal warfare killed an estimated 20,000 New Zealand Māori.

In 1848–49, as many as 40,000 out of 150,000 Hawaiians are estimated to have died of measles, whooping cough and influenza. Measles killed more than 40,000 Fijians, approximately one-third of the population, in 1875, and in the early 19th century devastated the Great Andamanese population. In Hokkaido, an epidemic of smallpox introduced by Japanese settlers is estimated to have killed 34% of the native Ainu population in 1845.

Concerns about future pandemics

Prevention of future pandemics requires steps to identify future causes of pandemics and to take preventive measures before the disease moves uncontrollably into the human population.

For example, influenza is a rapidly evolving disease which has caused pandemics in the past and has potential to cause future pandemics. The World Health Organisation collates the findings of 144 national influenza centres worldwide which monitor emerging flu viruses. Virus variants which are assessed as likely to represent a significant risk are identified and can then be incorporated into the next seasonal influenza vaccine program.

In a press conference on 28 December 2020, 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. The WHO and the UN, have warned the world must tackle the cause of pandemics and not just the health and economic symptoms.

The October 2020 'era of pandemics' report by the United Nations' Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, written by 22 experts in a variety of fields, said the anthropogenic destruction of biodiversity is paving the way to the pandemic era and could result in as many as 850,000 viruses being transmitted from animals—in particular birds and mammals—to humans. The "exponential rise" in consumption and trade of commodities such as meat, palm oil, and metals, largely facilitated by developed nations, and a growing human population, are the primary drivers of this destruction. According to Peter Daszak, the chair of the group who produced the report, "there is no great mystery about the cause of the Covid-19 pandemic or any modern pandemic. The same human activities that drive climate change and biodiversity loss also drive pandemic risk through their impacts on our environment." Proposed policy options from the report include taxing meat production and consumption, cracking down on the illegal wildlife trade, removing high-risk species from the legal wildlife trade, eliminating subsidies to businesses that are harmful to the natural world, and establishing a global surveillance network.

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.

Diseases with pandemic potential

There is always a possibility that a disease which has caused epidemics in the past may return in the future. It is also possible that little known diseases may become more virulent; in order to encourage research, a number of organisations which monitor global health have drawn up lists of diseases which may have pandemic potential; see table below.

Coronaviruses

Coronavirus diseases are a family of usually mild illnesses in humans include like that of the common cold but have resulted in outbreaks and pandemics such as the 1889-1890 pandemic, 2002–2004 SARS outbreak, '' Middle East respiratory syndrome–related coronavirus'' and the COVID-19 pandemic. There is widespread concern that members of the coronavirus family, particularly SARS and MERS have the potential to cause future pandemics. Following the end of the COVID-19 pandemic Public Health Emergency of International Concern deceleration by the World Health Organization, WHO director general Tedros Ghebreyesus stated he would not hesitate to re-declare COVID-19 a PHEIC should the global situation worsen in the coming months or years.

Influenza

The Greek physician Hippocrates, the "Father of Medicine", first described influenza in 412BC. It is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. Influenza may progress to pneumonia, acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease. In a typical year, 5–15% of the population contracts influenza. There are 3–5 million severe cases annually, with up to 650,000 respiratory-related deaths globally each year.

The first influenza pandemic to be pathologically described occurred in 1510. Since the pandemic of 1580, influenza pandemics have occurred every 10 to 30 years as the virus mutates to evade immunity. The 1889–1890 pandemic is estimated to have caused around a million fatalities, and the " Spanish flu" of 1918–1920 eventually infected about one-third of the world's population and caused an estimate 50million fatalities.

Tuberculosis

One-quarter of the world's current population has been infected with ''Mycobacterium tuberculosis'', and new infections occur at a rate of one per second. World Health Organization (WHO)
Tuberculosis Fact sheet No. 104—Global and regional incidence.
March 2006, Retrieved on 6 October 2006. About 5–10% of these latent infections will eventually progress to active disease, which, if left untreated, kills more than half its victims. Annually, eight million people become ill with tuberculosis, and two million die from the disease worldwide. Centers for Disease Control
Fact Sheet: Tuberculosis in the United States.
17 March 2005, Retrieved on 6 October 2006.

Viral hemorrhagic fevers

Viral hemorrhagic fevers such as Ebola virus disease, Lassa fever, Rift Valley fever, Marburg virus disease, Severe fever with thrombocytopenia as well as Argentine, Bolivian, Brazilian, Crimean-Congo and Venezuelan hemorrhagic fevers are highly contagious and deadly diseases, with the theoretical potential to become pandemics.

Yellow fever

Yellow fever has been a source of several devastating epidemics. Cities as far north as New York, Philadelphia, and Boston were hit with epidemics. In 1793, one of the largest yellow fever epidemics in U.S. history killed as many as 5,000 people in Philadelphia—roughly 10% of the population. About half of the residents had fled the city, including President George Washington.
Another major outbreak of the disease struck the Mississippi River Valley in 1878, with deaths estimated at 20,000. Among the hardest-hit places was Memphis, Tennessee, where 5,000 people were killed and over 20,000 fled, then representing over half the city's population, many of whom never returned. In colonial times, West Africa became known as "the white man's grave" because of malaria and yellow fever.

Zika virus

An outbreak of Zika virus began in 2015 and strongly intensified throughout the start of 2016, with more than 1.5 million cases across more than a dozen countries in the Americas. The World Health Organization warned that Zika had the potential to become an explosive global pandemic if the outbreak was not controlled.

Antibiotic resistance

Antibiotic-resistant microorganisms, which sometimes are referred to as "superbugs", may contribute to the re-emergence of diseases with pandemic potential that are currently well controlled.

For example, cases of tuberculosis that are resistant to traditionally effective treatments remain a cause of great concern to health professionals. Every year, nearly half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur worldwide. China and India have the highest rate of MDR-TB. The World Health Organization (WHO) reports that approximately 50 million people worldwide are infected with MDR-TB, with 79 percent of those cases resistant to three or more antibiotics. Extensively drug-resistant tuberculosis (XDR-TB) was first identified in Africa in 2006 and subsequently discovered to exist in 49 countries. During 2021 there were estimated to be around 25,000 cases XDR-TB worldwide.

In the past 20 years, other common bacteria including ''Staphylococcus aureus'', '' Serratia marcescens'' and '' Enterococcus'', have developed resistance to a wide range of antibiotics. Antibiotic-resistant organisms have become an important cause of healthcare-associated ( nosocomial) infections.

Climate change

Overpopulation

Encroaching into wildlands

Economic consequences

In 2016, the commission on a Global Health Risk Framework for the Future estimated that pandemic disease events would cost the global economy over $6 trillion in the 21st century—over $60 billion per year. The same report recommended spending $4.5 billion annually on global prevention and response capabilities to reduce the threat posed by pandemic events, a figure that the World Bank Group raised to $13 billion in a 2019 report. It has been suggested that such costs be paid from a tax on aviation rather than from, e.g., income taxes, given the crucial role of air traffic in transforming local epidemics into pandemics (being the only factor considered in state-of-the-art models of long-range disease transmission ).

The COVID-19 pandemic is expected to have a profound negative effect on the global economy, potentially for years to come, with substantial drops in GDP accompanied by increases in unemployment noted around the world. The slowdown of economic activity early in the COVID-19 pandemic had a profound effect on emissions of pollutants and greenhouse gases. Analysis of ice cores taken from the Swiss Alps have revealed a reduction in atmospheric lead pollution over a four-year period corresponding to the years 1349 to 1353 (when the Black Death was ravaging Europe), indicating a reduction in mining and economic activity generally.

See also

* Asian Flu of 1957
* Biological hazard
* Compartmental models in epidemiology
* Contagious disease
* Crowdmapping
* Disease X
* European Centre for Disease Prevention and Control (ECDC)
* Hong Kong Flu of 1968
* Immunization
* Index case
* Mortality from infectious diseases
* List of pandemics and epidemics
* Mathematical modelling of infectious disease
* Medieval demography
* Pandemic fatigue
* Pandemic severity index
* Public health emergency of international concern
* Super-spreader
* Syndemic
* Tropical disease
* Timeline of global health
* Twindemic
* Vaccination
* WHO pandemic phases

Notes

References

Further reading

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External links

WHO , World Health Organization

Past pandemics that ravaged Europe

Pandemic Influenza
at CDC

European Centre for Disease Prevention and Control

TED-Education videoHow pandemics spread

{{Authority control

Biological hazards
Doomsday scenarios
Economic problems
Future problems
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