Endemic (epidemiology)

{{See also|Mathematical modelling in epidemiology|Mathematical modelling of infectious disease}}

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

: $R\_0\; \backslash times\; S\; =\; 1$

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.{{Citation |last=von Csefalvay |first=Chris |title=Temporal dynamics of epidemics |date=2023 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780323953894000165 |work=Computational Modeling of Infectious Disease |pages=217–255 |publisher=Elsevier |language=en |doi=10.1016/b978-0-32-395389-4.00016-5 |isbn=978-0-323-95389-4 |access-date=2023-02-28}}

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.{{Cite journal |last1=Zhang |first1=Stephen X. |last2=Marioli |first2=Francisco Arroyo |last3=Gao |first3=Renfei |last4=Wang |first4=Senhu |date=2021-09-13 |title=A Second Wave? What Do People Mean by COVID Waves? – A Working Definition of Epidemic Waves |journal=Risk Management and Healthcare Policy |language=English |volume=14 |pages=3775–3782 |doi=10.2147/RMHP.S326051 |pmc=8448159 |pmid=34548826 |doi-access=free }}

It has been claimed that endemic COVID-19 implies that the disease severity would be mild. 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,{{cite journal |vauthors=Antia R, Halloran ME |date=October 2021 |title=Transition to endemicity: Understanding COVID-19 |journal=Immunity |type=Review |volume=54 |issue=10 |pages=2172–2176 |doi=10.1016/j.immuni.2021.09.019 |pmc=8461290 |pmid=34626549}} but in itself endemicity only means that there will be a steady, predictable number of sick people.

;Holoendemic: An endemic disease with an extremely high rate of infection, especially a disease that infects nearly everyone early in life, so that nearly all adults have developed some level of immunity.{{Cite book |last1=Porta |first1=Miquel S. |url=https://books.google.com/books?id=4kZmAwAAQBAJ |title=A Dictionary of Epidemiology |last2=Greenland |first2=Sander |last3=Hernán |first3=Miguel |last4=Silva |first4=Isabel dos Santos |last5=Last |first5=John M. |date=2014 |publisher=Oxford University Press |isbn=978-0-19-997673-7 |pages=136, 139 |language=en}}

;Hyperendemic: An endemic disease with a high rate of infection, especially one affecting people of all ages equally.

;Mesoendemic: An endemic disease with a moderate rate of infection.{{Cite book |last=Cockerham |first=William C. |url=https://books.google.com/books?id=WAnpCgAAQBAJ&dq=%22mesoendemic%22&pg=RA5-PA26 |title=International Encyclopedia of Public Health |date=2016-10-06 |publisher=Academic Press |isbn=978-0-12-803708-9 |pages=26–27 |language=en}} 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.{{Cite book |url=https://books.google.com/books?id=YXZyEAAAQBAJ&dq=%22mesoendemic%22&pg=PA11 |title=WHO Malaria Terminology, 2021 update |date=2021-11-24 |publisher=World Health Organization |isbn=978-92-4-003840-0 |pages=11 |language=en}}

;Hypoendemic: An endemic disease with a low rate of infection. Typhoid fever is a hypoendemic disease in the US.{{Cite book |last1=Emch |first1=Michael |url=https://books.google.com/books?id=ZCXSDAAAQBAJ&dq=%22hyperendemic%22+%22common+cold%22&pg=PA22 |title=Health and Medical Geography, Fourth Edition |last2=Root |first2=Elisabeth Dowling |last3=Carrel |first3=Margaret |date=2017-02-20 |publisher=Guilford Publications |isbn=978-1-4625-2006-0 |pages=22 |language=en}}

;Sporadic: A disease that appears occasionally, but, unlike endemic disease, is not always present at a steady and predictable level.{{Cite book |last=Battersby |first=Stephen |url=https://books.google.com/books?id=q0WTDAAAQBAJ&dq=%22hyperendemic%22+%22common+cold%22&pg=PA415 |title=Clay's Handbook of Environmental Health |date=2016-07-01 |publisher=Routledge |isbn=978-1-317-38291-1 |pages=415–416 |language=en}}

;Outbreak: An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event. The 2019–2020 measles outbreaks showed a normally endemic disease causing an epidemic outbreak, primarily among unvaccinated people.

;Epidemic: A new disease that is spreading or a previously endemic disease whose infection rate is increasing significantly. Seasonal flu frequently appears as an epidemic.

;Pandemic: An epidemic affecting a very large part of the world, generally multiple countries or multiple continents. Seasonal flu is sometimes a global pandemic.

Infections that are usually considered endemic include:

• Common cold{{cite journal |vauthors=Katzourakis A |title=COVID-19: endemic doesn't mean harmless |journal=Nature |volume=601 |issue=7894 |pages=485 |date=January 2022 |pmid=35075305 |doi=10.1038/d41586-022-00155-x |bibcode=2022Natur.601..485K |s2cid=246277859 |quote=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.}}
• Lassa fever
• Malaria
• Polio
• Rotavirus
• Hepatitis C
• Measles

Smallpox was an endemic disease until it was eradicated through vaccination.

The word endemic comes from the Greek: {{lang|grc|ἐν}}, {{transl|grc|en}}, "in, within" and {{lang|grc|δῆμος}}, {{transl|grc|demos}}, "people".{{cn|date=May 2023}}

• Syndemic – when two or more public health problems coincide and exacerbate each other
• Eradication of infectious diseases – when an infection declines until it no longer exists
• Vaccine-preventable diseases

• {{Wiktionary-inline|endemic}}

{{Concepts in infectious disease}}

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Category:Epidemiology

Category:Infectious diseases