Incidence and Prevalence are two important parameters of epidemiology, which not only help us to measure the burden of the diseases but also help us to understand the disease behavior which intern helps us control the diseases.

## Table of Contents

- Incidence and prevalence both are measures of morbidity.
- Prevalence
- Point prevalence
- Period prevalence
- Numerator and denominator in Incidence and prevalence
- How to Calculate Incidence
- Look carefully at your Denominator! In Incidence and Prevalence Calculation
- How to calculate Prevalence in epidemiology?
- Remember incidence is a measure of risk while prevalence is the measure of the disease burden!
- FAQs

### Is it possible to have a disease where the incidence rate is going down but the prevalence rate is going up?

## Incidence and prevalence both are measures of morbidity.

Incidence and prevalence both are important __measures in epidemiology.__

**Incidence** refers to the occurrence of new cases of disease or injury in a population over a specified period of time. Although some epidemiologists use incidence to mean the number of new cases in a community, others use incidence to mean the number of new cases per unit of the population.

**Two types** of incidence are commonly used — incidence proportion and incidence rate.

## Prevalence

Prevalence, sometimes referred to as prevalence rate, is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time. Prevalence differs from incidence in that prevalence includes all new and pre-existing cases in the population at the specified time, whereas incidence is limited to new cases only.

## Point prevalence

**Point prevalence** refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.

## Period prevalence

**Period prevalence** refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attributes at any time during the interval.

## Numerator and denominator in Incidence and prevalence

The numbers above the line are known as the numerator and the ones below the line are known as the denominator.

It’s up to you if you want to calculate the rate per thousand then multiply by 1000, or 100 if you want it in percentage!

## How to Calculate Incidence

for example in a group of 10 women who were free of disease, 3 develop uterine cancer in a year, so there will be a 3 divided by 10 multiplied by 1000, giving an incidence rate of 300 cases per 1000 population per year.

It is important to note that every person in the denominator must have the potential to be a part of the numerator. In our example these three women have transitioned from a healthy to a diseased state, Incidence is a measure of events, and therefore a measure of risk. So we said that everyone in the denominator must have the potential to become a member of the disease group.

## Look carefully at your Denominator! In Incidence and Prevalence Calculation

Let us come back to our example of uterine cancer, suppose the two women have a history of hysterectomy, then they will no longer have the risk of developing uterine cancer and so they have to be removed from the denominator. Recalculating incidence, now instead of 3/10, it would be 3/8 multiplied by 1000 equals the new incidence rate of 375 per 1000 population.

## How to calculate Prevalence in epidemiology?

As prevalence is defined as all new plus old cases in the total population at a given point in time. In our previous example if one was the old case then the prevalence would be 4 divided by 10, times 1000, giving a prevalence of 400 cases per 1000 population.

## Remember incidence is a measure of risk while prevalence is the measure of the disease burden!

Let’s understand this with an example.

Coronary heart disease prevalence in two cities compared, one is Mumbai and the other is Indore.

In this hypothetical example, Mumbai has a prevalence of 60 cases per 1000 population and Indore has a prevalence of 10 cases per 1000 population.

It looks like the risk of disease is higher in Mumbai!

Well! It’s not. Because prevalence is not a measure of risk but it shows the disease burden in the community.

Let’s look at the incidence rate and duration of disease in both locations.

The incidence rate is five per 1000 per year in both places, but the duration of the disease is 12 Years in Mumbai due to better care, whereas in Indore people die within two years of CHD.

So higher prevalence in Mumbai is just because of the better care prolonging life.

**Also, remember Prevalence = Incidence X Duration.**

## FAQs

**What is the difference between incidence and prevalence?**

Incidence refers to the number of new cases of a disease or condition that occur in a specific population over a certain period of time. Prevalence, on the other hand, refers to the total number of cases of a disease or condition in a specific population at a given point in time.

**How is the incidence rate calculated?**

The incidence rate is calculated by taking the number of new cases of a disease or condition that occur in a specific population over a certain period of time and dividing it by the total number of individuals in the population at risk during that same period of time.

**How is prevalence calculated?**

Prevalence is calculated by taking the total number of cases of a disease or condition in a specific population at a given point in time and dividing it by the total number of individuals in the population at that same point in time.

**What are some factors that can affect incidence and prevalence rates?**

Factors that can affect incidence and prevalence rates include changes in population size, demographic factors (such as age and gender), lifestyle factors (such as smoking and diet), and access to healthcare.

**How can incidence and prevalence be used in public health?**

Incidence and prevalence rates can be used in public health to determine the burden of a disease or condition on a population, to identify high-risk groups, and to evaluate the effectiveness of interventions or treatments.

**Are incidence and prevalence rates the same for all populations?**

No, incidence and prevalence rates can vary depending on the population being studied. Factors such as age, gender, lifestyle, and access to healthcare can all affect incidence and prevalence rates.

**What is the relation between incidence and prevalence?**

Prevalence is related to the incidence and duration of a disease or condition. A mathematical relationship that is often used to estimate prevalence from incidence is:**Prevalence = Incidence x Duration**

Where:

Incidence is the number of new cases of a disease or condition that occur in a specific population over a certain period of time.

Duration is the average length of time that an individual remains affected by the disease or condition.

This relationship is based on the assumption that the duration of the disease or condition is constant for all individuals and that there is no recovery or mortality. In reality, the duration of the disease or condition can vary from person to person, and recovery or death can occur.

It’s important to note that this equation is an approximation and that prevalence can be affected by other factors such as the duration of the disease, the recovery rate, and the mortality rate. In general, a high incidence rate over a short period of time will result in a high prevalence, and conversely, a low incidence rate over a longer period of time will result in a low prevalence.