What’s the difference between incidence and prevalence?

In the study of health conditions and disease, prevalence and incidence are methods of calculating the number of people who have a particular illness or condition. People often use the terms interchangeably, but they don't mean the same thing.

The difference between incidence and prevalence is vital. Which term you use depends on whether you’re calculating the percentage of people affected by a specific condition and whether cases are increasing or declining.

Understanding the difference between incidence and prevalence, and when each method of calculation is used, can help you determine which approach best aligns with your studies.

Prevalence is the number of individuals within a specific population with a health condition or disease at a defined time. It's a calculation of the total existing cases in a particular population.

Since prevalence is a changing number and not uniform across different populations, the time frame and the population must be defined for accuracy.

Depending on the circumstances, the population may consist of people in a specific location or those in a specific group (like students in a certain school). The time frame can also vary depending on how rapidly the condition is changing. It could be as short as a month or as long as a decade.

What is the best example of prevalence?

At the height of flu season, a school district may need to determine whether to close schools due to an influenza outbreak. If 20% of students in elementary schools in a certain district have seasonal influenza, the school may decide to close for a week to slow the spread.

To determine the prevalence, 500 students in an elementary school are tested. If 100 of those students have the flu, the prevalence is 20% and schools in the district will be closed for a week.

What is the formula for prevalence?

Prevalence can be expressed as a percentage or as a number. Since it's calculated using a specific group of people out of a larger population, you have two groups to use in the formula. For example, in a population of 10,000 people, 700 people are affected by a specific disease.

To determine prevalence as a percentage, divide the number of cases by the total population and multiply the result by 100.

Formula: (Number of cases during a specified period/total population) x 100

For the above example, the calculation is: 700/10,000 x 100 = 7%. This means seven percent of the population in the study have the disease.

To calculate prevalence as a specific number per thousand, for example, divide the number of cases by the total population and multiply the result by 1,000.

Formula: (Number of cases during a specified period/total population) x 1,000

The calculation for our above example is: 700/10,000 x 1,000 = 70. This means that for every 1,000 individuals, 70 people have the disease.

Is prevalence only related to disease?

Prevalence is the proportion of the population affected by a condition. While the term is often used concerning disease, it can be employed to describe other health characteristics like risk factors or lifestyle data.

For example, the prevalence of smokers may be noted in a study group of patients with cancer.

Incidence is the number of people who develop a health condition or disease within a particular time frame. While prevalence measures all instances of the condition in the study, incidence only calculates the number of new cases.

Within the same time frame, incidence is a part of the population that makes up prevalence. However, this calculation also provides essential information about the behavior of any health condition or disease.

The incidence of a contagious illness is an indicator of how fast the condition is spreading. In a non-transmissible disease, it can be a signal that the behaviors, ages, or other factors in a group are affecting the increase or decline of illness in the population.

What is the formula for incidence?

Incidence values are usually defined as a rate per number of the population. To evaluate the incidence rate, you need:

• The number of new cases

• The size of the population

• A specific time duration

To calculate the incidence rate, multiply the at-risk population by the time frame and divide the number of new cases by the result.

Formula: Number of new cases during a specified period/(population x time frame) 

If there are 3,000 new cases during one year in a population of 10,000, the calculation looks like this: 3,000/(10,000 x 1) = 0.3. In other words, three new cases per 1,000 people within the year.

Why is incidence more important than prevalence?

Prevalence includes the total number of people affected by a condition. In situations where the condition is treatable but not curable, survival influences growing prevalence. Since incidence only measures new cases, the number can reveal changing data that can be used to inform health protocols and procedures.

Prevalence is the percentage of people in a population affected by a health condition or disease. It includes the number of new cases and those that began outside the study time frame. Prevalence can change during the specified time duration due to recovery, deaths, or people moving outside the population.

Incidence is used to specifically identify new cases within a certain time frame. The rate doesn't change due to variations in the condition after the onset. As a result, incidence and prevalence don't always line up. When a vaccine is developed for an incurable condition, the incidence will be low but prevalence will still be high due to pre-existing cases.

The study of health conditions and diseases is a complex combination of the factors that cause the condition and the impact of treatment. Researchers rely on measures of prevalence and incidence to get an accurate picture of the evolution of a disease.

Depending on the circumstances surrounding a given condition, one measure may be used more than the other. For example, prevalence data may be reported more often for chronic diseases because individuals remain in the prevalence groups for long periods. The data can then be used to measure the success of treatments.

Incidence may be heavily relied on to report data on shorter, contagious illnesses, like COVID-19. Although they have different applications, incidence and prevalence are crucial in the assessment of risks and burdens of disease.