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For an infection that depends on person-to-person transmission to be endemic, each person who becomes infected with the disease must pass it to another person on average.
In epidemiology, an infection is said to be endemic (from the Greek ἐν for “inside” and “demonstrations” for “people”) in a population when that infection is constantly maintained at a baseline level in a geographic area without external inputs.
For example, chickenpox is endemic (steady state) in the UK, but malaria is not. Every year, some cases of malaria are 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).
While it might be common to say that AIDS is “endemic” in Africa, that is, found in one area, this is a use of the word in its etymological rather than epidemiological form.
AIDS cases in Africa are increasing, so the disease is not in an endemic state of equilibrium. It is correct to call the spread of AIDS in Africa an epidemic .
Assuming a completely susceptible population, this means that the basic reproduction number (R0) of the infection must be equal to 1. In a population with some immune individuals, the basic reproduction number multiplied by the proportion of susceptible individuals in the population ( S) must be 1.
This takes into account the probability that each individual to whom the disease can be transmitted is susceptible to it, effectively discounting the immune sector of the population. So, for a disease to be in an endemic state of equilibrium, it is: R0 x S = 1.
In this way, the infection is not extinguished nor does the number of infected people increase exponentially, but the infection is said to be in an endemic state.
An infection that begins as an epidemic will eventually die out (with the possibility of resurfacing in a theoretically predictable cyclical fashion) or reach endemic steady state, depending on a number of factors, including the virulence of the disease and its mode of transmission. .
If a disease is in an endemic steady state in a population, the above relationship allows us to estimate the R0 (an important parameter) of a particular infection. This, in turn, can be fed into the mathematical model of an epidemic.
What is an outbreak?
When there are more cases of a disease than expected in an area, an outbreak is declared. The area could be a small community or spread over several countries.
An outbreak could be a single case of a communicable disease new to a community or not seen for a long time. Outbreaks can last a few days, weeks, or even several years. There are three types of endemic outbreaks, an epidemic or a pandemic.
What is an epidemic?
An epidemic will cause a disease to spread rapidly among large numbers of people in a given population. During an epidemic, the disease will usually spread in two weeks or less.
There have been 14 epidemics since 2010, including the Ebola epidemic in West Africa, which killed 11,300 people between 2013 and 2016. In 2003, the SARS outbreak was classified as an epidemic, killing nearly 800 people.
What is a pandemic?
A pandemic is the worldwide spread of a new infectious disease . It spreads over a larger area, infects more people, and causes more deaths than an epidemic.
There have been a series of devastating pandemics in history, including smallpox , tuberculosis, and the Black Death, which killed more than 75 million people in 1,350. In 2009, a swine flu pandemic killed 14,286 people worldwide. .
When an emerging disease becomes endemic
Epidemics, such as HIV in the early 1980s and Ebola in 2014, inspire decisive investment and action by government and individual and societal concern, sometimes on the brink of panic.
In contrast, endemic diseases, such as HIV in 2017 and tuberculosis, struggle to maintain the same attention. For many, the paradox is that endemic disease as a whole continues to impose a much higher public health burden than epidemic disease.
In general, the rapid political response to the epidemics has been successful. It has been shown that epidemic diseases can be eradicated, often without the availability of vaccines and other biomedical technologies.
In recent times, only HIV has made the transition from epidemic to endemic, but diseases that have existed for centuries continue to cause most of the infectious disease burden.
The characterization of a disease as epidemic or endemic is commonly understood epidemiologically, but we argue that the social framing of a disease as “epidemic” or “endemic” is equally important.
For cattle, the relationship between disease phase and social response becomes more explicit: diseases are defined as “exotic” and thus acquire a legal status that requires government action to eliminate them.
Other endemic or livestock ‘production diseases’ are allowed to remain endemic. In contrast, in human health, disease status and responses are rarely legally determined.
As with animal diseases, human diseases acquire a social status, mainly based on their perceived risk, which determines their acceptability and the level of intervention that is considered appropriate.
In this exploration of the characteristics of epidemic and endemic infectious diseases, we highlight the fundamental role of risk and risk perception in explaining individual and social responses to diseases at different stages of their establishment in populations.
The heterogeneity of risk in populations is one of the key aspects that we consider. We argue that individual and societal risks determine, and are determined by, the classification of a disease as epidemic or endemic. Therefore, the classification of the disease reflects the biological and social phenomena.
The public response
Responses to epidemics tend to be public, seemingly without resource constraints, and often combine the efforts of national and global public health institutions.
For example, HIV led to a more than 10-fold increase in development financing for health over a decade and the creation of new public institutions, such as the Joint United Nations Program on HIV / AIDS (UNAIDS).
As diseases become endemic, they become more and more tolerated, and the place of responsibility may change to the individual.
Instead of public authorities actively detecting cases and subsidizing protection against risks, people can increasingly be encouraged to pay for the means to manage their own risks and seek care.
Similarly, the focus of any global response can shift away from direct service provision by international agencies to other forms of intervention, such as overall national capacity building, supported by national funding.
For many, the primacy of the national and global response to epidemics appears to be an “overreaction”, as resources are mobilized through government departments to quickly control and limit the outbreak.
Therefore, responses can be perceived as detrimental to other health priorities, such as routine vaccination. Epidemics can provoke broad multisectoral responses led by the political executive, mass information campaigns, and military mobilization.
Epidemics can also generate substantial public investment in vaccines or treatment development: the UK government is estimated to have spent £ 1.2 billion on the swine flu epidemic and licensing arrangements for diagnostics, drugs and vaccinations in emergencies.
The rationale for the political imperative is twofold. First, epidemics can be effectively controlled (financed) when the number of cases is very small, and therefore even a poorly informed rapid response can be more beneficial and efficient than a cautious response.
Second, there is a risk (often highly uncertain) of catastrophic impact: epidemics have destroyed civilizations. Therefore, politicians must weigh the investment against the highly uncertain but potentially devastating social, health and economic consequences.
The “public health paradox” ensures that, if the epidemic is successfully controlled, it is highly likely that the final impact of the epidemic disease will have been less than the opportunity cost of resources allocated from other health areas.
The 1918 experience still serves to remind us that if we had not responded to the H1N1 virus as we did, it could have been much, much worse.
At some point, the sociopolitical response to an emerging disease begins to change. Investment in disease can be institutionalized in the health sector, with those who recognize the importance of broader social factors and the impact of infectious diseases struggling to mobilize other sectors.
For example, despite the long-recognized association between poverty and tuberculosis (TB), combined health and social intervention remains a rarity, and is now pioneering in countries such as South Africa, where tuberculosis has become embedded as the leading cause of disease. death.
Funding may stabilize or decline, in part because other disease areas or the health system are recovering from any temporary loss of funding caused by the epidemic.
Resources for response are increasingly based on known risks and benefits that can be more clearly compared to alternative investments in the health sector or beyond, articulated in disease-specific “investment cases”.
Health insurance organizations, which can predict risk at this stage, can begin to cover any response on your insurance benefits (and premiums).
The disease itself takes on an ‘identity’, and interest groups are formed outside of populations where the disease is becoming endemic, often advocating for attention and action.
The disease must now compete for care and resources with other endemic diseases, even if the benefits of disease control clearly outweigh the costs.
The private answer
Individual responses to a disease drive the public response. On a personal level, epidemics can be very panicky because the risk of infection is impossible to measure and treatments are limited.
As individuals act to avoid risks (often largely unknown), they are willing to behave in ways that can have substantial social and economic costs and consequences.
People can avoid work, take children out of school, and run away or minimize travel. This reaction and its consequent costs are often not uniformly transmitted between populations.
The ability of an individual to act is limited by economic and social circumstances, and therefore, even at an early stage, epidemics begin to differentially impact different groups in society, as has been documented for the epidemic of the virus of the Ebola in Liberia.
Stigmatization of those considered to be most at risk of infection and transmission is common.
As epidemics morph into endemic disease, people gain the perception that they understand the risks of infection, which gives them a sense of control.
People cope with risk by adjusting behavior and mitigating the consequences, often to the point where new behaviors become a tolerable part of life.
The movement from the government’s place of responsibility to the individual can then be made possible by the belief that people can now make informed decisions, even when those people are severely limited by their circumstances.
For example, new forms of financing the means of protection against risks may emerge, such as the social marketing of condoms or mosquito nets for malaria, ultimately sharing the financial burden of the disease, even among the poorest.
As a disease becomes endemic, governments generally still provide some funds for treatment, but in the context of limited resources, governments may stop funding universal access, leaving many to turn to private care, including when treatment as in the case of tuberculosis or HIV.
For many stakeholders, this is a “lack of reaction” compared to the epidemic response.
Even when the disease has established itself in populations, these groups can continue to uphold the political imperative associated with epidemics, calling for urgent action to end the disease, unfortunately often with limited success.
The determinants of risk
The risk and its perception that drive the individual and social response to disease are a combination of the probability of infection together with the consequences of infection: the widespread fear associated with epidemics is often due to the lack of effective treatment.
In 2017, most of us would prefer an HIV diagnosis to an Ebola virus, but the distinction would have been less clear before 1995, when fewer effective drugs were available.
Epidemic diseases tend to have higher mortality and morbidity than endemic diseases, due to lack of clinical experience and knowledge, as well as innate pathogenicity. Over time, effective prevention and treatment interventions emerge.
However, while improved treatment is clearly a good thing, and reduces the risk of catastrophic loss of individual health, the reduction in risk can also cause a decline in political interest, initiating a more endemic response.
Where antimicrobial resistance emerges, reducing the treatability of endemic diseases like tuberculosis, fear can return and once again inspire a more epidemic response.
Paradoxically, the amount of public funds required to respond to infectious diseases may increase during a transition from epidemic to endemic, as organizations that made the initial investments in medical technologies to prevent, diagnose, and treat disease seek to recoup their investment.
Resource estimates for HIV were initially moderate but, as effective treatment emerged, they increased substantially.
Similarly, the availability of more effective treatment can increase costs for households, as the disease becomes chronic rather than acute, leaving households bearing the long-term cost of the disease and accessing to the atenttion.
For some population groups, unable to pay the costs of access to treatment, this can worsen the endemic and entrenched nature of the disease by reinforcing the cycles between risk and poverty, which can exacerbate the initial differentiation of risks that arose in the stage epidemic.
Lack of investment and political will allow diseases to take hold in certain population groups, and diseases become very costly and difficult to control, eliminate and eradicate.
In general, immunization is considered essential for eradication campaigns, although the roundworm is likely to be the first infection eradicated without resorting to immunization.
Global elimination and eradication of tuberculosis, malaria, and (now) HIV appears to be a long way off, but the biomedical tools we have for these diseases are effective, abundant, relatively inexpensive, and offer great potential. to reduce the burden of disease substantially.
These tools are notably better than those available for severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and Ebola, all of which have been eliminated with success.
The difference is in the political will to provide the necessary funds at the required scale. Failure to do so reflects the intrinsic acceptance of society that populations should tolerate the risk of infection in a way that they do for other health interventions.
However, people do not have an equivalent risk of acquiring or transmitting infections in epidemic or endemic stages. As epidemics progress, infection is increasingly associated with socially defined groups, who are at increased risk of infection, and generally a small proportion of the population.
However, behaviors that increase the risk of infection for some groups are not unique to those groups and, as the disease becomes endemic, the importance of the high-risk groups that predominated in epidemics is reduced.
For example, at the beginning of the HIV epidemic, people with the highest rates of sexual partner change are more likely to be infected and transmit faster – they are critical to the progression of the epidemic.
As HIV becomes more endemic, people with the same behavior remain at high individual risk for HIV infection, but most transmission can come from individuals with lower rates of sexual partner change.
This is for two reasons. First, the number of infected individuals with lower rates of sexual partner change is much greater than the number of people with high rates of sexual partner change.
Second, the status of partners is different: people at high risk are at higher risk because a large proportion of their partners are already infected, while people with lower rates of sexual partner change are more likely to have partners than yet. are not infected.
Consequently, as the infection spreads through populations, the risk of acquisition and the rate of transmission are less correlated.
Therefore, as epidemic diseases change to endemic diseases, although it takes root in specific groups, it can also move towards Rose’s ‘prevention paradox’: that is, small and common risks are responsible for more diseases than big and rare risks.
Also, people can become infected in one group and transmit to another. For example, older male partners can transmit HIV to adolescent girls and young women, who then infect their male partners.
It is no longer the case that funding can be intensively targeted at small, high-risk groups. Groups at higher risk of infection are increasingly dissociated from those at higher risk of spread.
This complicates the question of allocating scarce resources for endemic diseases: should they be allocated to those most likely to transmit or to those most likely to be infected?
In the early stages of epidemics, populations at risk are generally considered constant, and births, deaths, and the life course of behavior are ignored, which can lead to transitions between risk groups.
For example, commercial sex workers are at high risk of HIV infection, but relatively little consideration is given to their risks before entering this category, or after they leave.
In the transition to endemicity, social and economic transitions remain critical to understanding how infection spreads in a population and the dynamics of individual risks of infection and transmission.
Actions taken by individuals to tackle their poverty could lead to the spread of the disease, for example, the movement of sex workers around festivals in India, or miners in South Africa with tuberculosis returning home.
Migrant populations, such as refugees, may also have poor access to protection against risks, as they are often the groups least covered by health and social insurance.
Thus, the evolving risk of infection during endemicity can be more easily understood and predicted in terms of structural risk factors, that is, the economic, political and social factors that determine the size of these groups and the exposure of individuals. infection and the ability to access prevention and treatment services.
Over time, inequalities in risk can take hold, reflecting social structures and constraints, and, as with other forms of inequity, become essentially tolerated.
In the UK, it is accepted that tuberculosis is associated with homelessness and recent immigration and that HIV is associated with men who have sex with men.
The disease often continues to be endemic in specific population groups despite the availability of interventions to reduce transmission (for example, condoms, microbicides) and effective treatments, reflecting the social and economic constraints to access and use means of prevention and prevention. treatment.
We have argued that, in the broader public realm, epidemics and endemics are distinguished by individual and societal perceptions and tolerance for risk.
At the public level, we have the endemic diseases that we do because widespread multisectoral action, eradication or elimination is not politically attractive enough, even though the investment is usually profitable.
The restrained public response and the simultaneous shift of responsibility to individuals to protect themselves from risk means that endemic disease becomes more embedded, as those at risk are often the same people who do not have the private resources to avoid risk or access risk. treatment .
The endemic diseases that we have are those that have found a “niche” in social geography, and they often reinforce that niche. The labels “endemic”, “intractable” and “not eradicable” are self-fulfilling descriptions of established endemicity.
Actions to address the social and structural determinants of infectious diseases are increasingly being integrated into the response to both epidemic and endemic diseases.
The better the infrastructure and systems for preparedness and rapid responses to epidemics, the better we can avoid the unnecessary expenses and costs associated with rapid action in the face of uncertain, high-risk consequences.
Encouragingly, the use of entrenched approaches in anthropology and the social sciences are now beginning to be integrated into the infrastructure of early epidemic responses, and may improve response effectiveness and prevent the emergence of endemicity in specific population groups.
In the case of diseases that are endemic, the policy is increasingly recognized that a combined social and biomedical response is central to any elimination strategy.
Encouraging examples of successful combined social and biomedical intervention, such as scaling up community-based HIV prevention in India.
However, much remains to be done before social intervention is a common component of responses to infectious diseases.
Recognizing that epidemics and endemics are intrinsically defined socially and epidemiologically is a critical step in developing the comprehensive response required to address the complex challenge of eliminating infectious diseases.
