Basic concepts

Overview

Teaching: 5 min
Exercises: 0 min

Questions

• What is epidemiology and how does it contribute to global health?

Objectives

• By the end of this episode you will be able to recall definitions of basic terms used in epidemiology

What is Epidemiology?

In the following box we present a widely accepted definition that we will use in this course.

Definition: epidemiology

Epidemiology is the study of the distribution and determinants of the frequency of health-related states in specified populations, and the application of this study to the control of health problems.

We will clarify what we mean by these new terms, like “population”, “health-related state”, “frequency” and “distribution” in the next few paragraphs, but for now note that epidemiology is concerned with populations and health.

To better understand how epidemiology contributes to society we need to consider its role in public health: a multidisciplinary field whose goal is to promote the health of the population through organised community efforts. In contrast to medicine, which focuses mainly on treating illness in individuals, public health focuses on preventing illness in the community. Key public health activities include assessing the health status of the population, diagnosing its problems, searching for the causes of those problems, and designing solutions for them. The solutions usually involve community-level interventions that control or prevent the cause of the problem. For example, public health interventions include establishing educational programs to discourage teenagers from smoking, implementing screening programs for the early detection of cancer, and passing laws that require automobile drivers and passengers to wear seat belts.

The scientific basis of public health activities mainly comes from:

1. the basic sciences, such as pathology and toxicology;
2. the clinical or medical sciences, such as internal medicine and paediatrics;
3. the public health sciences, such as epidemiology, environmental health science, health education, and behavioural science.

Research in these three areas provides complementary pieces of a puzzle that, when properly assembled, provide the scientific foundation for public health action. Other fields such as engineering and economics also contribute to public health. The three main areas approach research questions from different yet complementary viewpoints, and each field has its own particular strengths and weaknesses. Thus Epidemiology is a science that provides one of several sources of knowledge for public health.

The rapid growth in the scope and ability to link multiple large static and streamed data sets such as electronic health care records provides opportunities and challenges for epidemiologists. As data sets become increasingly larger and more complex, epidemiologists will need advanced skills to link, manage, map, analyse, interpret, display, and communicate their findings to both lay and professional audiences and in both oral and written formats. Genomics is also entering the domain of epidemiological research; genome wide association studies can reach as many as 200,000 participants, and through these studies investigators address many of today’s epidemiological questions.

Populations

Above we have seen that epidemiologists study how disease can arise in populations. Now we look at what exactly we mean by a population. In the following we present a common definition, one that we wil use in this course.

Definition: population

A population is a set of people that share one or more common characteristics, such as gender, place of residence, age, or use of certain medical services.

For example, the set of all women under 50 living in Abuja is a population. The set of all people with the homozygous sickle cell mutation HbSS is another example of a population. A simple and useful measure of a population is its size: the number of people in the group. Estimates of a population size can often be obtained from census records or health registries.

It can be sometimes difficult to estimate the size of a population, especially if the the geographic spread is large, and it can also be a challenge to define the right population for a given study. Imagine you decide to study the effects of exercise for heavy smokers on their life expectancy. How we chose to define “heavy smoking” will affect who is considered for the study (i.e. the population) and who is not.

A unifying framework for thinking about a population is whether its membership is permanent or transient. A population whose membership is permanent is called a fixed population. Its membership is always defined by a life event. For example, the people in were in Hiroshima, Japan, when the atomic bomb exploded at the end of World War II are members of a fixed population. This population will never gain any new members because only people who were at this historical event can be members. A dynamic population is a population who’s membership can change. Recall the populations we discussed above: the set of all women under 50 living in Abuja is clearly a dynamic population, since women are born, immigrate, emigrate, or die each day and this changes the population size.

Question

Is the set of all HbSS patients in the world a fixed or dynamic population?

Answer

Dynamic. Clearly no patients can migrate in or out of the population because it covers the whole world, but it is certainly dynamic because new HbSS children are born every day.

It is often helpful to talk about a population defined by a given healthcare facility. For example, the set of all people that use the Groote Schuur medical hospital in Cape Town, South Africa, defines a population. The catchement area of a medical facility such as the Groote Schuur hospital may be of interest to policy makers. A catchment population of a facility is a population that consists of the people who use the facility’s services.

We may also use the concept of a candidate population. A candidate population is a populaiton whose members are all identified as being at high risk of some adverse health event. This could be the possible contraction of an infectious disease, some people in malaria endemic regions could be a candidate population for malaria, or it could be people at risk or cardic arrest. Candidate populaitions are easy to identify for high risk conditions, but cancer for example is relatively low risk, so candidate populations for cancer studies are difficult to identify.

In conducting an epidemiology study, it is rare that we will be able to examine or monitor every member of a population. In practice, we need to obtain a sample that faithfully represents the population. A sample of a population is a subset of the population where each member in the sample is chosen at random from the population, so that the statistics of the sample are suitably close to the statistics of the population. The question of whether or not the sample is “suitably close” is an important issue for statisticians that must be treated with care.

Health-related states

Epidemiology was initially a study of epidemics, where infectious diseases would spread rapidly through a population in a short period of time. The success of the early work in the field led to its slow adoption into other areas of health. Soon epidemiologists were also looking at endemic diseases in their populations, diseases that had been a near constant problem for these people for some time. The field’s scope continued to expand, and epidemiologists began looking at non-communicable dieases, chronic diseases, and then the heath effects of lifestyle choices like smoking. The introduction of seatbelts in cars came about after the publication of results from car accident studies. Now epidemiologists look at everything from optimal exercise routines to genetic modifiers of chronic diseases.

We will use the term health-related state to capture this wide variety of topics. A health-related state of a member of a population is a state of a person that is health-related. This can be anything which can be described as a “state”, or a characteristic of a person that is related to their health. For example, an individual who is a heavy smoker is a health related state. An individual with a history of stroke is another example. These states are specifically of an individual so as to allow epidemiologists to count affected members of a population. A characteristic of a population for example is not a valid health-related state.

We caution that epidemiologists can sometimes use the word “disease” to refer to the more general concept of a health-related state. Since this can be confusing we will not use this terminology but be aware that in other texts the term disease can have this more general meaning.

What are the goals of Epidemiology?

Let us revisit our epidemiology definition. It mentions the frequency, distribution, determinants, and control of health-related states. We will look at these terms in more detail now.

A Frequency measure, often reffered to simply as a frequency, is a measure of how often a health-related state arises in a population. That is, roughly, the number of people in a population who can be confirmed as being in that state (having contracted disease X, or having been in a car accident). For a given state or population there are often several ways to define a frequency. We will encounter many such examples in this module.

Counting, which is a key activity of epidemiologists, includes three steps:

1. developing a definition of disease,
2. instituting a mechanism for counting cases of disease within a specified population,
3. and determining the size of that population.

National disease registries, for example the Sickle Pan African Research Consortium (SPAN) Nigeria database, are a key resource for estimating the frequency of disease in a population.

Distribution is the change in frequency of a health-related state in a population with respect to the characteristics of person, the place, or time. It relates, in other words, to who is getting the disease, where it is occurring, and how it is changing over time. Variations in frequency by these three characteristics provide useful information that helps epidemiologists understand the health status of a population; formulate hypotheses about the determinants of a disease; and plan, implement, and evaluate public health programs to control and prevent adverse health events. In the SPAN Nigeria database, the locations of each patient is recorded along with clinical information, thus allowing epidemiologists to analyse how the frequency of the disease varies with location.

Question

List the 3 dimensions in which the frequency of health-related states of members of a population may change

Answer

Time, place, and characteristics of person

Determinants are factors that bring about a change in a person’s health-related status. Thus, determinants consist of both causal and preventive factors. Determinants also include individual, environmental, and societal characteristics. Individual determinants consist of a person’s genetic makeup, gender, age, immunity level, diet, behaviours, and existing diseases. For example, the risk of breast cancer is increased among women who carry genetic alterations, such as BRCA1 and BRCA2; are elderly; give birth at a late age; have a history of certain benign breast conditions; or have a history of radiation exposure to the chest. Within the population of sickle cell patients, several important factors have been known to affect their quality of life, such as co-inheritance of alpha-thalassemia, or mutations in genes related to production of fetal haemoglobin.

Epidemiologists accomplish disease control through epidemiological research, as described previously, and through surveillance. The purpose of surveillance is to monitor aspects of disease occurrence that are pertinent to effective control. For example, the Centres for Disease Control and Prevention collects information on the occurrence of HIV infection across the United States. For every case of HIV infection, the surveillance system gathers data on the individual’s demographic characteristics, transmission category (such as injection drug use or male-to-male sexual contact), and diagnosis date. These surveillance data are essential for formulating and evaluating programs to reduce the spread of HIV.

Epidemiologists therefore define count members of populations and asses the frequency of disease in these populations. They look at how this frequency can change from place to place, or time to time. Medical and biological studies can assist with working out what factors might be influencing the disease, and then control measures can be introduced that either address the areas where the frequency is highest, or in a way that the most important determinants can be be eliminated.

Frequency measures

We have mentioned that there can often be many ways to define a frequency measure for a given population or health-related state. Different measures have different strengths and weaknesses. The two most common types of measure are incidence and prevelance.

The incidence of a health-related status is the proportion of a candidate population that changes their given health-related state over a specified period of time. The important part of this definition is that it pertains to candidate populations. This means, if we are interested in describing the onset of a particular disease in a population, we migt consider to estimate the incidence.

The prevelance of a health-related state is the proportion of the total population that has the given health-related state. In this second type of frequency measure notice we do not specify a time period, or a candidate population. This meansure is more useful for describing the burden of a given health-related state in a population, as for example might be useful when preparing national budgets for health facilities. For example, A goverment might want to estimate the prevelance of sickle cell anemia in Dar es Salaam so that we can allocate sufficient funds to help local facilities support their patients.

The measure of prevelence has been adapted to include temporal components, which may be useful for certain studies. We can estimate the prevelence of a given state at an instant in time, or over a period of time. The point prevalence of a health-related-state is the proportion of the population that has a given health-related state. at a single point in time. The period prevalence of a health-related state is the proportion of the population that has a given health-related state during a specified duration of time.

References

1. Ann Aschengrau and George R. Seage III, Essentials of epidemiology in public health, Fourth edition (2020, Jones & Bartlett Learning)
2. https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section1.html
3. https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section4.html

Key Points

• Epidemiology is one of several sources of knowledge for Public Health.

• Electronic health records and genome wide association studies provide sources of Big Data for today’s epidemiologists.

• A population is a group of people that share a common characteristic.

• A population can either be fixed, or dynamic. Useful examples include catchment populations and candidate populations

• Members of a sample are chosen at random from a population, so that the statistics of the sample are suitably close to the statistics of the population.

• frequency measures estimate how many members of a population have a certain health-related state.

• The geographical or temporal variation of frequency is called distribution.

• Disease control can be achieved through surveillance programs.

• Newborn screening programs and national disease registries are important epidemiological contributions to sickle cell disease management.