Incidence & Prevalence

1. Prevalence & Incidence: Definitions

Definition of Prevalence (Harvard School of Public Health)

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Definition of Incidence (Harvard School of Public Health)

2. Understanding Epidemiologic Concepts

Epidemiology is the underlying and basic science of Public Health. It could be defined as any research of health events in populations, including:

How many are affected by such events?
Is the risk increasing or decreasing?
What is the relevance of the problem?
How could it be prevented?

2.1. Definitions

Health event: A particular disease, injury, other health condition or attribute.
Population: The total number of persons living in a particular place (e.g. a town or country), or the total number of persons in a particular group (e.g. with the same job or educational background).
Prevalence: The total number of cases of a health event in a specified population.
Prevalence rate: The proportion of a population that has a health event:

at a specified point in time (e.g. on a certain date) – ‘point prevalence’, or
during a specified period (e.g. over 12 months) – ‘period prevalence’.

Proportion: A ratio between health events occurring and population, often shown as a fraction or percentage. For example, the number of people who have a disease compared with the total number of people studied.
Incidence: The number of new cases of a health event during a given period in a specified population.
Incidence rate: The frequency with which new health events occur, related to a particular time frame (e.g. the number of new cases per year). Incidence rate is worked out by dividing the number of new cases over a specified period either by:

the average population (usually ‘mid period’ - the population half-way through the period being looked at), or
‘person-time’ - a measure of the number of persons at risk and the time they were at risk.

For example, in a study, the average population would be the number of participants in the study at the half way point. Person-time is the number of participants in the study, accounting for the amount of time that they were actually in the study.

2.2. How Many People Are Affected?

Usually the first question about a certain health event will be: How many people are affected? Or what is the prevalence?

Depending on the population we are looking at, the answer will differ. Let’s say that there are 10,000 cases in country A and 20,000 cases in country B. Could we state that the health problem is far more serious in country B? Let’s have a look on the following table:

	Country A	Country B
Persons affected by a certain health event	10.000	20.000
Total population of the country	20.000	200.000

In country A, a total of 10,000 out of a population of 20,000 are affected, which means that 50% are affected by the health event.

In country B, a total of 20,000 out of a population of 200,000 are affected, which means that 10% are affected by the health event.

The mathematical way to calculate this would be:

$The % of population affected with a health event is the number of persons affected divided by the total population mult 100$

With this formula we get information about the percentage of a population that is affected by a health event.

While our first impression would be that 20,000 are twice as many as 10,000 (which is definitely true!), we now have a very different picture, as we put the numbers in relation to the population.

Understanding these basics will make the concept of prevalence easier to understand. The prevalence compares the number of persons having a certain characteristic with the total population.

2.3. Example - Hair Colour

Imagine that a while ago you were celebrating a party with friends. You look back at a group photo and notice that some of your friends have their hair coloured red. There were 100 guests and 30 of them had red hair – the prevalence of red hair at the party was 30%.

If we were talking about a health event, we would ask: ‘What is the prevalence of that health event?’, but the mathematical calculation would be exactly the same. We could express the prevalence as percentages (here 30%) or alternatively as cases per 1,000 persons (here 300 per 1,000).

3. Prevalence

In a population of 10,000 patients having a certain disease, 50 persons are reported to be affected by another disease in addition. This is called a ‘co-morbidity’. So what is the prevalence of the co morbidity in this population?

The mathematical way to calculate this would be:

Number of cases divided by the population. The result multiplied by 100

This formula will provide us with the information as a percentage. By dividing 50 by 10,000 and multiplying the result by 100 (to make it a percentage) we find out that 0.5% of the population is affected. So the prevalence of the co-morbidity is 0.5% in our population.

Apply the formula with 50 cases and population 10.000. Result 0.5%

Rather than expressing prevalence as a percentage, we can describe it as the number of people affected in a standard sized population, for example 1,000 people. So instead we would calculate:

Apply the formula with 50 cases and population 10.000. Standard size population of 1000. Result 5

This means that, for each 1,000 patients, five of them have co-morbidity.

Keep the example of the party in mind. Prevalence is like describing a group photo:

How many people can you see there? That number is your population.
How many people share a certain feature (e.g. same hair colour)? This number is used to calculate prevalence.

3.1. Calculating Prevalence

In epidemiology we actually have three different ways to calculate the prevalence:

Point prevalence: the number of cases of a health event at a certain time. For example, in a survey you would be asked if you are currently smoking.
Period prevalence: the number of cases of a health event in reference to a time period, often 12 months. For example, in a survey you would be asked if you have smoked during the past 12 months.
Lifetime prevalence: the number of cases of the health event in reference to the total lifetime. For example, in a survey, you would be asked if you have ever smoked.

4. Incidence

Prevalence looks at existing cases, while incidence looks at new cases. Reading in a newspaper that a certain country has a prevalence of 2% of a certain disease will not give you any information about when those patients got the disease, or for how long they have had it.

For example, HIV is nowadays a treatable infection with a normal life expectancy. This means that with stable numbers of new cases, prevalence numbers will increase. Looking at the new cases (incidence) provides a deeper understanding of what is going on.

Depending on how incidence is expressed, either as a percentage or per person-year there are two different formulas.

To express incidence as a percentage:

Incidence is equal to new cases during period of observation divided by total population at risk during the same period

To express incidence per person-year at risk:

Incidence is equal to new cases during period of observation divided by time at risk

Often you can read about the incidence of a heath event as a certain number of cases per 100 or 1,000 person-years.

In such a case, the epidemiologist would count the total years the (healthy) population was at risk and divide the new cases by this amount of person-years at risk. This is described in more detail below.

4.1. Calculating Incidence

Look at the following table describing a 5-year study of a population of five patients. We are following these five healthy patients, as we would like to analyse the incidence of disease ‘A’.

	Year 1	Year 2	Year 3	Year 4	Year 5
Patient 1	Healthy	Healthy	Healthy	No data	No data
Patient 2	Healthy	‘A’	Healthy	Healthy	Healthy
Patient 3	Healthy	Healthy	Healthy	Healthy	Healthy
Patient 4	Healthy	Healthy	Healthy	‘A’	Healthy
Patient 5	Healthy	Died

Patient 1 was followed for three years and after his third annual check he did not show up any more. We do not know why. He did not get the disease ‘A’ in the first three years and we do not know what happened afterwards. We will keep him in our records with ‘3 person-years at risk’.

Patient 2 was followed for five years and got sick in the second year but recovered. How many years at risk will we include? One or four? This actually depends on the type of the disease. We assume in our example that you could get the disease more than once in your lifetime, so we would include him with ‘4 person-years at risk’.

Patient 3 has been followed for five years and never got the disease; he was ‘5 person-years at risk’.

Patient 4 has got the disease in the fourth year. We will include four years at risk into our calculation as we said earlier that you could get re-infected with the disease in this example.

Patient 5 was followed for one year and died (of an unknown cause). We do not know any details regarding disease ‘A’. Only the one year in which they were at risk (i.e. healthy) is added to the cumulative total of person-years at risk.

We found two new cases in a population of five persons, so we could say that the incidence was:

2 (cases) divided by 5 (population) multiplied by 100. The result is 40%

We might speak of an incidence of 40 new cases per 100 persons. We can also calculate the incidence using healthy years instead of total population. In this case we divide the number of new cases (two cases) by the total of the healthy years where persons are at risk to the disease (17 years).

2 (new cases) divided by 17 (total of healthy years at risk to the desease). Multiplied by 1000. Result 117.6

The result of 117.6 would be read as ‘117.6 new cases per 1,000 person-years’.

Site:	EUPATI Open Classroom
Course:	Statistics
Book:	Incidence & Prevalence

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Date:	Saturday, 27 April 2024, 3:06 AM

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