Prevalence vs. Incidence
Image by: Lexi Wang
What are Prevalence and Incidence?
Prevalence and incidence are two ways to measure population characteristics like disease, trait or behavior.
Prevalence is the commonness of a characteristic. It measures the proportion of a population who has the characteristic at a specific moment in time.
Incidence is the rate of occurrence. It describes the number of new people who display the characteristic in a period of time.
Characteristic: a Breakdown
Prevalence and incidence can either describe a disease, trait or behavior.
A disease could be an infectious disease like Ebola or a chronic disease like obesity. A trait could range from a genetic mutation to a hair color. A behavior could be a risk factor like smoking or a preventative action like wearing a seatbelt.
Prevalence: An Example
Imagine we have a population of 1,000 people. If 100 people have blue hair the prevalence of blue hair in the population is 10%.
Incidence: An Example
We find out that out of the 100 cases of blue hair, only 10 occurred in the last month. Since incidence only looks at the new cases that occurred over a specific period of time, we would say that the incidence rate for blue hair over the past month is 1%.
Different but related
Although these measurements are different, they are related. Envision a sink and think of water as the measured characteristic. Prevalence will reveal how full the sink is at any given moment, while incidence will measure the inflow of water. The outflow of water will be how many people with the characteristic are cured or die.
Riddle Me This
Let’s look at the burden of HIV/AIDS in Kenya in the 1990s. Over 10 years, the prevalence of HIV/AIDS increased while the incidence of HIV/AIDS decreased. How is that possible? Think about what would need to happen to the sink...
In the 1990s, the Kenyan government ramped up education and other prevention programs, which decreased the incidence of new HIV/AIDS cases. Simultaneously, treatment became more widely available, and more people were able to live longer with HIV/AIDS.
Let’s bring this back to the sink analogy. While less water flowed in, therapeutics “plugged” the drain and the sink filled up.
*Just a reminder, HIV is the virus that can lead to AIDS.
Teamwork makes the Dream Work
Looking at prevalence and incidence together can tell us a much more comprehensive story.
Outside the Huddle
Prevalence and Incidence Defined | Obesity Prevention Source | Harvard T.H. Chan School of Public Health