Endemics, Epidemics and Pandemics

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Original Editor - Kim Jackson Top Contributors - Kim Jackson, Tony Lowe, Lucinda hampton, Tarina van der Stockt and Nicole Hills

Introduction

Infectious diseases are ever-present in society but every once in a while there may be an outbreak, a new strain or a new disease that has a significant impact at either a local or global level. The spread and rate of new cases can be classified as:

• Endemic - describes a disease that is present permanently in a region or population
• Epidemic - is an outbreak that affects many people at one time and can spread through one or several communities
• Pandemic - is the term used to describe the significant global spread of an epidemic.

Endemic

Endemic is derived from Greek en meaning in and demos meaning people. It is used to describe disease that is at a constant rate within a society or country. Each country may have a disease that is unique, for example in the Caribbean Dengue is still present due to failure to eradicate the Aedes aegypti mosquito. Dengue first appeared in the Americas and the Caribbean and with the assistance of the Pan American Health Organisation (PAHO) in the 1950s and 1960s the Americas were largely able to eradicate the presence of the Aedes Aegypti virtually eliminating the occurrence of Dengue. However, failure to eradicate its presence in the Caribbean resulted in the continued transmission throughout the region and more recently it has found its way back into the Americas causing several epidemics. Another example of an endemic is varicella, more commonly known as chickenpox in the UK. It is more common in children under the age of 10, who often only experience mild symptoms and after exposure build up a natural immunity to the virus. Although there is a vaccine available it is only offered to those who are seen as vulnerable. Malaria is another infectious disease that is endemic to Africa; through education and implementing countrywide strategies the cases of Malaria are now falling (see video below)

Epidemic

An epidemic is derived from Greek epi meaning upon or above and demos meaning people and is the term used to describe the rapid spread of disease to a large number of people in a given population over a short time period. The term epidemic is not used just purely to describe the increase of infectious diseases but it is often linked to any detrimental rise of health risks within a society. An example of this is the latest interest in the rising weight of children and adults. Over the last 3 decades, the United States has seen an increase in the number of people who have a BMI higher than the recommended average. The rise in obesity globally is termed as an obesity epidemic (expand). When the term is used in connection with infectious diseases it is due to the sudden rise of cases usually as the result of:

• A change in the host population, for example moving from animals to humans (zoonotic diseases)
• A genetic change in the infectious agent, eg bacteria, virus, fungi or parasite
• Introduction of new pathogens to a host population

Epidemics can follow trends and these trends are often used to control the spread of the infection. A typical example of this is seasonal flu.

Pandemic

An infectious disease can be spread by either bacterial or viral agent but in most cases a pandemic is the result of a transmissible infectious disease. Once an epidemic becomes global and affects a large percent of the population it becomes known as a pandemic. The terms pandemic and epidemic are used to describe the rate and distance of the spread of the disease and not the severity of the disease. Significant features of a pandemic are listed below:

• Affects a wider geographical area, often global
• Infects a large number of people
• Often caused by a new virus or a new strain of a virus that has been dormant for many years.
• Spreads quickly in humans as there is little to no immunity
• Can cause a high number of deaths
• Because of the need to control the spread of the disease, there is often social disruption, unrest and economic loss

Escalation of an Epidemic to a Pandemic

The World Health Organization will declare a Pandemic when a disease has shown exponential growth - dramatically increasing rate of growth, each day showing many more cases than the previous day. A current example of this is the Coronavirus Disease (COVID-19). On 31 December 2019, a cluster of cases of pneumonia of unknown cause, in the city of Wuhan, Hubei province in China, was reported to the World Health Organisation. This was subsequently identified as a new virus in January 2020 and over the following months, the number of cases continued to rise but were not contained to China and showed exponential growth worldwide. Due to the rapid global rise in cases this was declared a pandemic on 11 March and as of 15 March 2020, over 150,000 cases had been identified globally in 123 countries with over 5,000 fatalities

Stages of a Pandemic

The WHO has identified six phases that it follows before declaring a pandemic. Phase 1 represents a low risk and phase 6 is a full-blown pandemic, you can see the phases below:

• Phase 1 - A virus is seen in animals but has not been shown to infections in humans
• Phase 2 - An known animal virus has caused an infection in humans
• Phase 3 - Scattered or isolated incidence of cases or small clusters of the disease occuring in humans. Possible cases of human-to-human transmission but not a level to cause community-level outbreaks
• Phase 4 - Human to human transmission at a rate that causes an outbreak in communities
• Phase 5 - The spread of the disease between humans is now evident in more than one country
• Phase 6 - Community-level outbreaks are in at least one additional country other than that seen in phase 5.

One Phase 6 is reached preparation is then made for a global pandemic. Each phase has a list of actions that need to be followed to allow transparency and education of health organisations and members of the public.

Managing a Pandemic

Once a pandemic is identified it is vital to take the appropriate action to contain, manage and reduce the spread of the virus. The key message at this stage is reduce the transmission rate - the number of individuals infected by a single infected individual. If on average across a population the transmission rate is greater than one the number of cases will continue to increase. Measures that reduce the transmission rate to less than one will result in a decline in the total number of inflections.There are several measures that have proven effective in the control and containment of viruses:

• Initial containment (within a country) - actions designed to prevent the spread of the disease, making sure all the cases are identified, controlled, and isolated. quarantine self-isolation. Need to be well prepared and act early.
  • Identify cases - “very quickly limit people coming in, identify the sick, immediately isolate them, use heavy protective gear to protect their health workers, track all their contacts, quarantine them…” (https://medium.com/@tomaspueyo/coronavirus-act-today-or-people-will-die-f4d3d9cd99ca)
  • Trace contacts -
  • Quarantine - separate an individual suspected of infection from contact with others for a certain period of time that covers the period of incubation for the disease
  • Isolation - separate an individual who has been identified as infected from contact with others

If the above measures do not show a significant decrease in the number of cases reported then it may be necessary to seek more extreme measures to contain and decrease the transmission rate.

• Mitigation - reduce the rate of spread of the disease (not eliminate the spread) through social distancing (cancel events, closing institutions, work from home etc.)
• National containment: “border control from the air and sea, case identification (using new data and technology), quarantine of suspicious cases, proactive case finding, resource allocation (assessing and managing capacity), reassurance and education of the public while fighting misinformation, negotiation with other countries and regions, formulation of policies toward schools and childcare, and relief to businesses”

Containment measures used by Taiwan - Wang CJ, Ng CY, Brook RH. Response to COVID-19 in Taiwan: Big Data Analytics, New Technology, and Proactive Testing. JAMA. Published online March 03, 2020

The above measures all have the same purpose and that is to flatten the curve - reduce the peak of number of cases in order to maintain the ability of the healthcare system to provide quality care.  The greater the stress on the healthcare system the higher the mortality rate. Extending the time scale of the epidemic so that a potential vaccine can at some future point be used to limit to extent of the spread of the disease.