Epidemic Curves

00:00 Hello, in this video we'll tackle one of my favorite topics, the epidemic curve.

00:06 That's a thing you probably see a lot of on TV or in textbooks, the incidence rate going up and down over time.

00:12 So we'll go over some of the different kinds of epidemic curves and how we relate them to the different kinds of epidemics.

00:19 I hope you'll learn something and I hope you enjoy the content, thank you.

00:24 An epidemic curve shows the frequency of new cases over time.

00:29 So essentially, it's a graph and it shows over the course of time how many new cases or the incident rate that occurs over the course of an outbreak.

00:39 The shape of the curve in relation to the incubation period tells us some things about the source of the outbreak and maybe some characteristics about the nature of the disease and how it's going to progress through the population.

00:54 Always the horizontal axis, the X axis, that's the date or the time of illness onset and the vertical axis is the number of cases, the number of new cases for that period and the shape of the curve, the type of the curve is going to vary with the type of the epidemic.

01:15 So right away, just by looking at the shapes, we can know a lot about the nature of the outbreak.

01:23 Things that "Epi curves"can teach us include the time trend of the outbreak so the distribution of cases over time.

01:31 it can show us who the outliers are.

01:33 So some cases are going to be extraordinary, they'll one day may give us a very large number of cases, another day may give us a very small number of cases.

01:43 Those will be unusual occurrences.

01:46 It also would give us a general sense of the outbreak's magnitude.

01:49 How big is it? How long can we expect it to last? And maybe we can infer something about the pattern of spread, how it's moving through the population and something about the likely time of exposure.

02:04 One final thing an Epi curve might be able to give us hints about is, when we can expect the outbreak to be over and how well certain interventions are working as we apply them in real-time.

02:18 So if you remember our different kinds of outbreaks, we have a point-source epidemic.

02:23 Point-source epidemic is kind of like if you go to a picnic and someone brings tainted food, well that's the one source where everyone got sick from, it's a single point.

02:34 So a point-source outbreak tends to look like this.

02:38 It's a single wave.

02:41 So its cases increase over time and then they decrease a little bit more slowly.

02:48 And all the cases tend to fall within one incubation period of the pathogen.

02:52 So it's fairly easy to identify.

02:55 A continuous common source epidemic, however, rises to a peak and then falls.

03:01 So in this curve you can see it goes up and it goes down but it stays up a little bit longer.

03:08 That's because the infection is lasting a bit longer.

03:12 An example would be tainted water.

03:14 It's one common source that everyone in the community is drinking from and so many people are becoming infected over a period of time.

03:22 And eventually, the infections go away as we discover the source of the infection and close off that water source as it is.

03:30 So again, the shape here implies there's an ongoing source of contamination.

03:37 An intermittent common-source Epi curve looks like this And if you remember what that is, an intermittent common source epidemic would be something like a factory upstream from a village that's dumping pollutants into the water and that's causing people to get sick.

03:56 But it isn't dumping pollutants every single day or week, it's doing it sporadically, so maybe a few days out of the year it's doing so and people are getting sick and then it stops, then it happens again.

04:07 And when it dumps water into the river, it isn't for the same amount of time all the time.

04:12 So the epidemic curve caused by this kind of outbreak would look like waves, but the waves don't have to be the same width all the time.

04:21 So the onset is abrupt and the cases are spread over a great period of time.

04:26 The incubation period will depend on how long the exposure lasts, how long that source is producing toxin or infection.

04:36 A propagated epidemic is one that moves from person to person, like an infectious disease or respiratory disease.

04:44 and cases occur over more than one incubation period.

04:49 so they usually look like a series of waves.

04:51 That's why things like the flu or COVID-19 happen in waves.

04:58 It's not uncommon for the waves, the curves to be successively larger before coming down again.