Inepidemiology, aninfection is said to beendemic in a specificpopulation orpopulated place when thatinfection is constantly present, or maintained at abaseline 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 becomeepidemic.[3]
For example,chickenpox is endemic in the United Kingdom, butmalaria 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 suitablevector (mosquitoes of the genusAnopheles). 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.
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 thebasic reproduction number (R0) of the infection must equal one. In a population with someimmune individuals, the basic reproduction number multiplied by the proportion ofsusceptible individuals in the population (S) must be one. This takes account of theprobability of each individual to whom the disease may betransmitted being susceptible to it, effectively discounting the immune sector of the population. So, for a disease to be in anendemic steady state orendemic equilibrium, it holds that
In this way, the infection neither dies out, nor does the number of infected people increaseexponentially. 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 thevirulence of the disease and itsmode of transmission.[5]
If a disease is in an endemic steady state in a population, the relation above allows thebasic reproduction number (R0) of a particular infection to be estimated. This in turn can be fed into amathematical 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]
It has been claimed thatendemic 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]
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 ofimmunity.[9]
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]
An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event.[8] The2019–2020 measles outbreaks showed a normally endemic disease causing an epidemic outbreak, primarily amongunvaccinated people.[3]
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]
^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.ISBN978-1-5297-8581-4.Terms used to describe distribution include: Endemic: a condition that is generally present in a group or area, such as a cold
^abcdefghKatzourakis 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.PMID35075305.S2CID246277859.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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The dictionary definition ofendemic at Wiktionary
