Endemic (epidemiology)

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Human alphaherpesvirus 3 virion - a herpesvirus known to infect humans. It causes chickenpox (varicella), a disease most commonly affecting children, teens, and young adults, and shingles (herpes zoster) in adults. Varicella (Chickenpox) Virus PHIL 1878 lores.jpg
Human alphaherpesvirus 3 virion − a herpesvirus known to infect humans. It causes chickenpox (varicella), a disease most commonly affecting children, teens, and young adults, and shingles (herpes zoster) in adults.

In epidemiology, an infection is said to be endemic in a specific population or populated place when that infection is constantly present, or maintained at a baseline level, without extra infections being brought into the group as a result of travel or similar means. [1] The term describes the distribution of an infectious disease among a group of people or within a populated area. [2] An endemic disease always has a steady, predictable number of people getting sick, but that number can be high ( hyperendemic ) or low (hypoendemic), and the disease can be severe or mild. [3] [4] Also, a disease that is usually endemic can become epidemic. [3]

Contents

For example, chickenpox is endemic in the United Kingdom, but malaria is not. Every year, there are a few cases of malaria reported in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector (mosquitoes of the genus Anopheles ). Consequently, the number of people infected by malaria in the UK is too variable to be called endemic. However, the number of people who get chickenpox in the UK varies little from year to year, so chickenpox is considered endemic in the UK.

Mathematical determination

For an infection that relies on person-to-person transmission, to be endemic, each person who becomes infected with the disease must pass it on to one other person on average. Assuming a completely susceptible population, that means that the basic reproduction number (R0) of the infection must equal one. In a population with some immune individuals, the basic reproduction number multiplied by the proportion of susceptible individuals in the population (S) must be one. This takes account of the probability of each individual to whom the disease may be transmitted being susceptible to it, effectively discounting the immune sector of the population. So, for a disease to be in an endemic steady state or endemic equilibrium, it holds that

In this way, the infection neither dies out, nor does the number of infected people increase exponentially. An infection that starts as an epidemic will eventually either die out (with the possibility of it resurging in a theoretically predictable cyclical manner) or reach the endemic steady state, depending on a number of factors, including the virulence of the disease and its mode of transmission. [5]

If a disease is in an endemic steady state in a population, the relation above allows the basic reproduction number (R0) of a particular infection to be estimated. This in turn can be fed into a mathematical model for the epidemic. Based on the reproduction number, we can define the epidemic waves, such as the first wave, second wave, etc. for COVID-19 in different regions and countries. [6]

Misuse

It has been claimed that endemic COVID-19 implies that the disease severity would be mild. [3] However, endemicity has no inherent relationship with disease severity. Endemic COVID-19 could be mild if previously acquired immunity reduces the risk of death and disability during future infections, [7] but in itself endemicity only means that there will be a steady, predictable number of sick people. [3] [4]

Categories of endemic diseases

Holoendemic
An endemic disease with an extremely high rate of infection, [8] especially a disease that infects nearly everyone early in life, so that nearly all adults have developed some level of immunity. [9]
Hyperendemic
An endemic disease with a high rate of infection, [8] especially one affecting people of all ages equally. [9]
Mesoendemic
An endemic disease with a moderate rate of infection. [8] This term is often used to describe the prevalence of malaria in a local area, with 10 to 50% of children showing evidence of prior infection being considered a moderate level for that disease. [8] [10]
Hypoendemic
An endemic disease with a low rate of infection. [8] [9] Typhoid fever is a hypoendemic disease in the US. [11]

Categories for non-endemic diseases

Sporadic
A disease that appears occasionally, but, unlike endemic disease, is not always present at a steady and predictable level. [12]
Outbreak
An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event. [8] The 2019–2020 measles outbreaks showed a normally endemic disease causing an epidemic outbreak, primarily among unvaccinated people. [3]
Epidemic
A new disease that is spreading or a previously endemic disease whose infection rate is increasing significantly. [8] [11] Seasonal flu frequently appears as an epidemic. [8]
Pandemic
An epidemic affecting a very large part of the world, generally multiple countries or multiple continents. [8] Seasonal flu is sometimes a global pandemic. [8]

Examples

This is a short, incomplete list of some infections that are usually considered endemic:

Smallpox was an endemic disease until it was eradicated through vaccination. [13]

Etymology

The word endemic comes from the Greek: ἐν, en, "in, within" and δῆμος, demos, "people".[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Pandemic</span> Widespread, often global, epidemic of severe infectious disease

A pandemic is an epidemic of an infectious disease that has a sudden increase in cases and spreads 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.

<span class="mw-page-title-main">Infection</span> Invasion of an organisms body by pathogenic agents

An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

<span class="mw-page-title-main">Measles</span> Viral disease affecting humans

Measles is a highly contagious, vaccine-preventable infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than 40 °C (104 °F), cough, runny nose, and inflamed eyes. Small white spots known as Koplik's spots may form inside the mouth two or three days after the start of symptoms. A red, flat rash which usually starts on the face and then spreads to the rest of the body typically begins three to five days after the start of symptoms. Common complications include diarrhea, middle ear infection (7%), and pneumonia (6%). These occur in part due to measles-induced immunosuppression. Less commonly seizures, blindness, or inflammation of the brain may occur. Other names include morbilli, rubeola, red measles, and English measles. Both rubella, also known as German measles, and roseola are different diseases caused by unrelated viruses.

<span class="mw-page-title-main">Epidemic</span> Rapid spread of disease affecting a large number of people in a short time

An epidemic is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

<span class="mw-page-title-main">Herd immunity</span> Concept in epidemiology

Herd immunity is a form of indirect protection that applies only to contagious diseases. It occurs when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination, that the communicable pathogen cannot maintain itself in the population, its low incidence thereby reducing the likelihood of infection for individuals who lack immunity.

<span class="mw-page-title-main">Disease outbreak</span> Sudden increase in occurrences of a disease

In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire continent. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent. Outbreaks include many epidemics, which term is normally only for infectious diseases, as well as diseases with an environmental origin, such as a water or foodborne disease. They may affect a region in a country or a group of countries. Pandemics are near-global disease outbreaks when multiple and various countries around the Earth are soon infected.

<span class="mw-page-title-main">Basic reproduction number</span> Metric in epidemiology

In epidemiology, the basic reproduction number, or basic reproductive number, denoted , of an infection is the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection. The definition assumes that no other individuals are infected or immunized. Some definitions, such as that of the Australian Department of Health, add the absence of "any deliberate intervention in disease transmission". The basic reproduction number is not necessarily the same as the effective reproduction number , which is the number of cases generated in the current state of a population, which does not have to be the uninfected state. is a dimensionless number and not a time rate, which would have units of time−1, or units of time like doubling time.

In epidemiology, force of infection is the rate at which susceptible individuals acquire an infectious disease. Because it takes account of susceptibility it can be used to compare the rate of transmission between different groups of the population for the same infectious disease, or even between different infectious diseases. That is to say, is directly proportional to ; the effective transmission rate.

Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health and plant health interventions. Models use basic assumptions or collected statistics along with mathematics to find parameters for various infectious diseases and use those parameters to calculate the effects of different interventions, like mass vaccination programs. The modelling can help decide which intervention(s) to avoid and which to trial, or can predict future growth patterns, etc.

Compartmental models are a very general modelling technique. They are often applied to the mathematical modelling of infectious diseases. The population is assigned to compartments with labels – for example, S, I, or R,. People may progress between compartments. The order of the labels usually shows the flow patterns between the compartments; for example SEIS means susceptible, exposed, infectious, then susceptible again.

Pox parties, also known as flu parties, are social activities in which children are deliberately exposed to infectious diseases such as chickenpox. Such parties originated to "get it over with" before vaccines were available for a particular illness or because childhood infection might be less severe than infection during adulthood, according to proponents. For example, measles is more dangerous to adults than to children over five years old. Deliberately exposing people to diseases has since been discouraged by public health officials in favor of vaccination, which has caused a decline in the practice of pox parties, although flu parties saw a resurgence in the early 2010s.

A breakthrough infection is a case of illness in which a vaccinated individual becomes infected with the illness, because the vaccine has failed to provide complete immunity against the pathogen. Breakthrough infections have been identified in individuals immunized against a variety of diseases including mumps, varicella (Chickenpox), influenza, and COVID-19. The characteristics of the breakthrough infection are dependent on the virus itself. Often, infection of the vaccinated individual results in milder symptoms and shorter duration than if the infection were contracted naturally.

Globalization, the flow of information, goods, capital, and people across political and geographic boundaries, allows infectious diseases to rapidly spread around the world, while also allowing the alleviation of factors such as hunger and poverty, which are key determinants of global health. The spread of diseases across wide geographic scales has increased through history. Early diseases that spread from Asia to Europe were bubonic plague, influenza of various types, and similar infectious diseases.

<span class="mw-page-title-main">Sunetra Gupta</span> British novelist and epidemiologist

Sunetra Gupta is an Indian-born British infectious disease epidemiologist and a professor of theoretical epidemiology at the Department of Zoology, University of Oxford. She has performed research on the transmission dynamics of various infectious diseases, including malaria, influenza and COVID-19, and has received the Scientific Medal of the Zoological Society of London and the Rosalind Franklin Award of the Royal Society. She is a member of the scientific advisory board of Collateral Global, an organisation which examines the global impact of COVID-19 restrictions.

<span class="mw-page-title-main">Chickenpox</span> Human viral disease

Chickenpox, also known as varicella, is a highly contagious, vaccine-preventable disease caused by the initial infection with varicella zoster virus (VZV), a member of the herpesvirus family. The disease results in a characteristic skin rash that forms small, itchy blisters, which eventually scab over. It usually starts on the chest, back, and face. It then spreads to the rest of the body. The rash and other symptoms, such as fever, tiredness, and headaches, usually last five to seven days. Complications may occasionally include pneumonia, inflammation of the brain, and bacterial skin infections. The disease is usually more severe in adults than in children.

<span class="mw-page-title-main">Virgin soil epidemic</span> Worse effects of disease to populations with no prior exposure

In epidemiology, a virgin soil epidemic is an epidemic in which populations that previously were in isolation from a pathogen are immunologically unprepared upon contact with the novel pathogen. Virgin soil epidemics have occurred with European settlement, particularly when European explorers and colonists brought diseases to lands they settled in the Americas, Australia and Pacific Islands.

<span class="mw-page-title-main">Superspreading event</span> Event in which 3 or more people attend and an infectious disease is spread much more than usual

A superspreading event (SSEV) is an event in which an infectious disease is spread much more than usual, while an unusually contagious organism infected with a disease is known as a superspreader. In the context of a human-borne illness, a superspreader is an individual who is more likely to infect others, compared with a typical infected person. Such superspreaders are of particular concern in epidemiology.

<span class="mw-page-title-main">Ira Longini</span> American biostatistician

Ira M. Longini is an American biostatistician and infectious disease epidemiologist.

In epidemiology, the term hyperendemic disease is used to refer to a disease which is constantly and persistently present in a population at a high rate of incidence and/or prevalence (occurrence) and which equally affects all age groups of that population. It is one of the various degrees of endemicity.

<span class="mw-page-title-main">Endemic COVID-19</span> Theoretical future stage of COVID-19

COVID-19 is predicted to become an endemic disease by many experts. The observed behavior of SARS-CoV-2, the virus that causes COVID-19, suggests it is unlikely it will die out, and the lack of a COVID-19 vaccine that provides long-lasting immunity against infection means it cannot immediately be eradicated; thus, a future transition to an endemic phase appears probable. In an endemic phase, people would continue to become infected and ill, but in relatively stable numbers. Such a transition may take years or decades. Precisely what would constitute an endemic phase is contested.

References

  1. ↑ "Principles of Epidemiology in Public Health Practice, Third Edition An Introduction to Applied Epidemiology and Biostatistics". Centers for Disease Control and Prevention. Retrieved 19 April 2018.
  2. ↑ Cook, Neal; Shepherd, Andrea; Dunleavy, Stephanie; McCauley, Claire (23 April 2022). "Health and Disease in Society". Essentials of Pathophysiology for Nursing Practice. SAGE. pp. 114–115. ISBN   978-1-5297-8581-4. Terms used to describe distribution include: Endemic: a condition that is generally present in a group or area, such as a cold
  3. 1 2 3 4 5 Katzourakis A (January 2022). "COVID-19: endemic doesn't mean harmless". Nature. 601 (7894): 485. Bibcode:2022Natur.601..485K. doi:10.1038/d41586-022-00155-x. PMID   35075305. S2CID   246277859.
  4. 1 2 Ticona, Eduardo; Gao, George Fu; Zhou, Lei; Burgos, Marcos (13 April 2023). "Person-Centered Infectious Diseases and Pandemics". In Mezzich, Juan E.; Appleyard, James; Glare, Paul; Snaedal, Jon; Wilson, Ruth (eds.). Person Centered Medicine. Springer Nature. p. 465. ISBN   978-3-031-17650-0.
  5. ↑ von Csefalvay, Chris (2023), "Temporal dynamics of epidemics", Computational Modeling of Infectious Disease, Elsevier, pp. 217–255, doi:10.1016/b978-0-32-395389-4.00016-5, ISBN   978-0-323-95389-4 , retrieved 28 February 2023
  6. ↑ Zhang, Stephen X.; Marioli, Francisco Arroyo; Gao, Renfei; Wang, Senhu (13 September 2021). "A Second Wave? What Do People Mean by COVID Waves? – A Working Definition of Epidemic Waves". Risk Management and Healthcare Policy. 14: 3775–3782. doi: 10.2147/RMHP.S326051 . PMC   8448159 . PMID   34548826.
  7. ↑ Antia R, Halloran ME (October 2021). "Transition to endemicity: Understanding COVID-19". Immunity (Review). 54 (10): 2172–2176. doi:10.1016/j.immuni.2021.09.019. PMC   8461290 . PMID   34626549.
  8. 1 2 3 4 5 6 7 8 9 10 Cockerham, William C. (6 October 2016). International Encyclopedia of Public Health. Academic Press. pp. 26–27. ISBN   978-0-12-803708-9.
  9. 1 2 3 Porta, Miquel S.; Greenland, Sander; Hernán, Miguel; Silva, Isabel dos Santos; Last, John M. (2014). A Dictionary of Epidemiology. Oxford University Press. pp. 136, 139. ISBN   978-0-19-997673-7.
  10. ↑ WHO Malaria Terminology, 2021 update. World Health Organization. 24 November 2021. p. 11. ISBN   978-92-4-003840-0.
  11. 1 2 Emch, Michael; Root, Elisabeth Dowling; Carrel, Margaret (20 February 2017). Health and Medical Geography, Fourth Edition. Guilford Publications. p. 22. ISBN   978-1-4625-2006-0.
  12. ↑ Battersby, Stephen (1 July 2016). Clay's Handbook of Environmental Health. Routledge. pp. 415–416. ISBN   978-1-317-38291-1.
  13. 1 2 3 4 5 6 7 8 Katzourakis A (January 2022). "COVID-19: endemic doesn't mean harmless". Nature. 601 (7894): 485. Bibcode:2022Natur.601..485K. doi:10.1038/d41586-022-00155-x. PMID   35075305. S2CID   246277859. Yes, common colds are endemic. So are Lassa fever, malaria and polio. So was smallpox, until vaccines stamped it out. [...] learning to live with endemic rotavirus, hepatitis C or measles.
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