Incidence and prevalence—what you really need to know

If you want to understand the medical literature, you absolutely need to understand the difference between incidence and prevalence and what they have to do with one another. This video from our Epidemiology Essentials course is ideal for anyone who’s just diving into clinical epidemiology. As you will learn, incidence is a measure of risk whereas prevalence is a measure of disease burden. Both parameters have their strengths and should be used in different settings.

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[00:00:00] Hey, everyone. This video is about incidence and prevalence. It's absolutely crucial to understand what incidence and prevalence are and what the difference between the two is. Incidence is a measure of disease risk, whereas prevalence measures disease burden. Incidence is defined as the number of new cases of a disease, during a specific time period, divided by the number of persons at risk for the disease, during that same time period. The resulting proportion can be

[00:00:30] multiplied by 1 000, in order to get the number of new cases per 1 000 population or by 100 in order to get the number of new cases per 100 population. That's up to the beholder. Let's take the example of a fictitious population of ten women free from disease, four of these women developed uterine cancer during a given year. So, the incidence is 4, divided by 10, times 1 000, equalling an incidence rate of 400 cases per 1 000 population per year.

[00:01:00] What's critical for incidence is that every person in the denominator must have the potential to become one of the people in the numerator. In fact, these four people are part of the denominator. They transition from disease-free to diseased. 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 numerator or the disease group. Let's come back to our study of uterine cancer.

[00:01:30]If three women of our fictitious population had their uteruses removed, if they had undergone hysterectomy, in other words, they would no longer have the potential to become part of the numerator. So, instead of 4, divided by 10, it would now be 4, divided by 7, times 1 000, equals a new fictitious incidence rate of 570 per 1000. Now, what about prevalence? Prevalence is defined as the number of affected persons

[00:02:00] in the population, divided by the number of all persons in that population, at a specific point in time. So, in this fictitious example, it would be 3, divided by 10, times 1000, which would mean that we have a prevalence of 300 per 1000 population. And what's the difference between incidence and prevalence? Let's take a group of five people who developed tuberculosis over a period of six months. One develops the disease in January, two in February, one

[00:02:30] in April, and one in May. So, ignoring the denominator for now, the incident cases in February, for example, would be two. Or in other words, two would be the numerator of the incidence rate for that month and it would be one for the months of January, April, and May. Now, prevalence also takes into account the duration of the disease. Let's say case number one is cured in May, case number two is cured in April, case number three

[00:03:00] in March, case four and five during the second half of the year. Now, let's look at the prevalence of tuberculosis in these months. Let's also just focus on the numerator or the prevalent cases during each month. We have one case in January, three cases in February, three in March, three in April, three in May, and two in June. So, prevalence is determined by how many people joined the prevalence pool or the incidence and how many leave the prevalence pool, through death

[00:03:30] or cure. Let us depict that with a vessel full of pebbles. The vessel is filled with pebbles or new cases by the incidence rate and it's emptied by death or cure. This means that if death goes up then more people will leave the vessel, and if death goes down then more people will stay in the vessel and the same thing applies for cure. If more people are cured, more pebbles or people will leave the vessel. And cure rate goes down, then more pebbles

[00:04:00] or people will stay in the vessel. Let's have a closer look. Let's say this is our baseline prevalence. Now, what happens to prevalence if incidence goes up while death and cure stay the same? It has to go up, right? What happens if incidence stays the same but death goes down while cure rate stays the same? It will go up since fewer pebbles will leave the vessel. What if death goes down and cure rate goes up? Well, as you probably guessed correctly, prevalence will decrease since more pebbles will leave the vessel.

[00:04:30] So, prevalence is a measure of disease burden, as such, it is a great tool for planning the allocation of health services. So, prevalence is incidence, times duration. And duration is determined by the death rate or the cure rate. If the death rate or cure rate go up then the disease duration will be shortened. If death rate and cure rate go down, disease duration will be prolonged. At most points in time, prevalence will be lower, with shorter duration and higher,

[00:05:00] with longer duration of disease. Now, let's use another fictitious example to clarify this point. Let's use an example of coronary heart disease in two different populations. One is the population of Hollywood, the other one is the population of the Bronx. Our fictitious Hollywood has a prevalence of 50 cases per 1000 population. The Bronx, on the other hand, has a prevalence of 10 per 1000. So, to the person who's not familiar with the fact that prevalence is not a measure of

[00:05:30] risk but rather a measure of the burden of the disease. It looks like folks from Hollywood are at an increased risk of CHD but that conclusion is not true at all here. Let's look at the incidence rates, duration of disease in both locations. Incidence is 5 per 1000, per year, in both locations but duration of disease is 10 years in Hollywood, due to better care, whereas in the Bronx, people die from CHD after 2 years. So, in fact, the higher

[00:06:00] prevalence in Hollywood is just an expression of better care of an incurable disease. You see, we said prevalence is equal to incidence, times duration. So, 5 times 10 is 50, and 5 times 2 is 10. So, I hope the difference between incidence and prevalence is now crystal clear. Now, I would love to hear your thoughts in the comment section below.