Understand the difference between outbreak, epidemic and pandemic

by ace
Understand the difference between outbreak, epidemic and pandemic

When many cases of a contagious disease start to be reported, headlines soon appear talking about a particular outbreak, epidemic and the risk of a pandemic. However, it is not always clear what each of these levels is and exactly when it moves from one stage to the next.

The first important concept is that of endemic. This is a certain number of cases that have historically occurred in a certain region of the country. Brazilian examples: Chagas disease and schistosomiasis (water belly).

When this endemic level (which may be 0) is broken by the increase in cases, it can be considered that there is an outbreak or an epidemic.

Generally speaking, there is an “outbreak” to designate that new cases are concentrated in a certain region, such as a neighborhood in a city or a metropolitan region.

The word “epidemic” is usually reserved for when the geographical delimitation (a village or a neighborhood, for example) no longer helps to define as well where the cases of the disease are happening and / or when many people are affected.

The distinction is somewhat gray, but an infection that may help to illustrate the problem is measles. Recent measles outbreaks killed 140,000 people in 2018 alone, according to WHO. It is estimated that measles epidemics in the 1960s killed 2.5 million people.

When the epidemic affects several countries or continents, it is a pandemic. One case or another of a disease outside the place where the outbreak initially occurred does not necessarily imply a pandemic. Other factors, such as spreading capacity of the infectious agent (as in the flu virus) and the presence of a vector (Aedes aegypti mosquito, in the case of arboviruses such as dengue and zika) contribute to the containment or spread of the disease.

But when exactly does a major epidemic turn into a pandemic? How many countries have to be affected? In what proportion? Does the severity of the disease matter?

There is a consensus that the Spanish flu, which a hundred years ago killed at least 50 million people, can be called a pandemic. The swine flu outbreak in 2009, which killed 200,000 people worldwide, is also said to have been a pandemic.

On a article published in The Journal of Infectious Diseases, in 2009, the authors, including Anthony Fauci, director of Niaid (National Institute of Allergy and Infectious Diseases of the USA) reflect on what would be necessary to attest to this extreme level:

  • Large geographical distribution: uOne consensus is that the disease has to affect a large portion of the territory, as in the case of the black plague, influenza (influenza) and HIV / AIDS.
  • Traceability of the movement of the disease: it is possible to identify the path taken by the disease, as in the case of influenza, transmitted through the respiratory tract, cholera, water, or dengue, which occurs according to the presence of vectors (mosquitoes of the genus Aedes).
  • High infection rate: when the transmission rate is low or there is a low proportion of symptomatic cases, a disease is rarely treated as a pandemic, even with widespread dissemination. West Nile fever left the Middle East and ended up in Russia and the West in 1999, but it never carried the nickname epidemic
  • Low population immunity: The chance of a pandemic is greater when the population's immunity is low for the pathogen
  • New: the use of the term pandemic is associated with the risk of new pathogens (case of HIV, in the 1980s) or new variants (case of influenza virus, influenza, which presents new configurations seasonally)
  • Infectiousness: the term “pandemic” is less commonly linked to non-infectious diseases, such as obesity, or risky behavior, such as smoking. When this occurs, the idea is to highlight that problem as an area that deserves attention, but, according to the authors of the article, it is a colloquial use, not so scientific.
  • Type of contagion: most cases of epidemics are diseases transmitted between people, such as influenza (influenza).
  • Gravity: generally the word “pandemic” is associated with serious diseases, capable of killing, such as black plague, HIV / AIDS and SARS (severe acute respiratory syndrome). But less severe diseases, such as scabies (caused by a mite) or acute hemorrhagic conjunctivitis (caused by viruses), were also considered pandemics.

The main way to prevent the effects of a pandemic is with surveillance systems to quickly detect cases, have laboratories equipped to identify the cause of the disease, have a team qualified to contain the outbreak, avoiding new cases and management systems. crisis, to coordinate the response.

The WHO (World Health Organization), in turn, uses specific terms to classify certain situations. An emergency occurs when an authority decides that it is time to take extraordinary measures, such as travel and trade restrictions and quarantine. That same authority can also suspend this state of emergency. An emergency is usually well defined in time and space and depends on a certain threshold to be declared. This threshold can be defined as a mortality rate of 1 for every 10,000 people per day or mortality of 2 children under 5 years for every 10,000 people per day.

Crisis is a situation classified as difficult, difficult to study, classify and combat. A crisis may not necessarily be evident and needs analysis work to be fully known and combated.

Other sources consulted: Ministry of Health, Fredi Alexander Diaz Quijano (Faculty of Public Health – USP), CDC

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