So
last time we looked at the definitions of incidence and prevalence, and I asked you to think in your heads about water going into a bucket (incident cases) and the water that was already in it (prevalent cases). If the spigot on the bucket were opened, the water would go out, but there would still be the same rate of water coming in. (Prevalent cases remained constant, if the spigot let out as much water as the water that came in.) If the spigot is closed, then the prevalent cases continue to rise and rise. That's the case if those infected never recover (or die) from the disease.
Anyway, enough with the mental exercises. Let's look at some examples, all after the jump...
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| Looks like an incidence curve |
EXAMPLE NUMERO UNO
Our first example is a simple one. It's one that we see every year. The flu comes and goes in waves every year. Some years are better than others for a myriad of reasons. I can, however, guarantee to you that, if vaccination rates were better, each wave would be smaller and sharper. That is, the flu would come and go quickly and with fewer cases. So let's look at what the incident and prevalent cases look like, and then what the incidence and prevalence rates look like.
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| Seriously, if more people vaccinated, those curves would be flatter, shorter |
As you can see, the number of cases has peaks and troughs as the flu seasons come and go. Both the number of new (incident) cases and existing (prevalent) cases go up and down. Everyone in this population is at risk of contracting the flu each and every year. (Except for those whose flu vaccine confer immunity, of course.) Trust me when I tell you that the graph for the number of cases per 100,000 looks exactly the same. So, no big deal here. It's pretty straight forward.
EXAMPLE NUMERO DOS
This example is what you would see if the number of new cases was the same each year and no one died from the disease or condition.
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| If only we saw 100 cases of HIV each year and no one died... If only... |
As you can see, the number of incident cases is constant at 100 new cases each year. However, the prevalent cases continue to increase each year, because the cases don't die. But what would the proportions look like under these conditions?
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| Looks familiar, doesn't it? |
Well, it looks pretty much the same, with one exception. Can you see it? I had to add the data labels to show that incidence is slowly climbing. Yes, the incidence is slowly going up, even with 100 new cases each and every year, and nothing more. Why?
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| "¿Por qué?" is Spanish for "Why?" |
Remember, as new cases are coming along, the population at risk begins to decline. From part one of this lesson, you know that the population at risk is the denominator of the fraction. As the fraction gets smaller, the proportion gets higher. It's math. One fifth is smaller than one fourth. One fourth is smaller than one third. And one half is bigger than them all.
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