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  • 00:01

    SPEAKER 1: Epidemiological curves, what are they?How do you use them?How do you make them?This is the second in a two-part video series.In the first video, we talked about howto make an epidemiological curve using Microsoft Excel.If you haven't watched that, go back and watch it right now.And in this video, we're going to talkabout how to interpret them, how to make sense of them.And of course in a revised briefing,what epidemiological curves are.

  • 00:22

    SPEAKER 1 [continued]: So firstly, what isn't epicurve or an epidemic curve?An epidemic curve is a visual representationof the onset of illness of cases in an outbreak over time.We're going to talk about three kinds of outbreaks.The first is a point source outbreakand that's when a population or a group of peopleare exposed to a pathogen at a single source,

  • 00:43

    SPEAKER 1 [continued]: at a point in time, or over a very brief period of time.That's kind of like if a group of people attend a weddingand there's some dodgy food, and they allget sick a few days later.And the second is a common source outbreak.And this is when people are exposedto some sort of environmental hazardthat takes place over a period of time,like if there's pathogens in a water source.

  • 01:03

    SPEAKER 1 [continued]: And finally, we're going to talk about propagated outbreaks.And that happens when there's an infectious agent thatspreads from person to person.So if you look at the example that's on the screen right now,that's a point source outbreak.Everybody was exposed at one point in time.Now, you'll notice in this example,that despite the fact that everybody got exposedat the same point in time, they all attended the same wedding,they all eat the same food, not everybody got sick

  • 01:25

    SPEAKER 1 [continued]: at the same time.The reason for this is that there'sa lot of variation between people in terms of firstly,the extent of their exposure.So some people might have had more of the cake than others,and their individual susceptibility.Now the period of time between peoplebeing exposed to the pathogen and actually becoming illis called the incubation period.For each infectious agent, we knowthat there's a period of time during which nobody gets ill,

  • 01:45

    SPEAKER 1 [continued]: even though they've been exposed.We call that the minimum incubation period.And of course there's a maximum incubation period.And that's a period of time after which nobody gets ill.So the epidemic curve really is a distributionof incubation periods.Now let's imagine that this wasn't a point source outbreak.Let's imagine that this was a common source outbreak.And basically that means that the population

  • 02:06

    SPEAKER 1 [continued]: was exposed to the pathogen over a period of time,like for example, if the water source had pathogens in it.So the important thing here in terms of epidemic curves,is that the population is exposedfor an extended period of time.And remember that in a point source outbreak,the people continue to get sick until the endof the maximum incubation period.So in a common source outbreak, because the period of time

  • 02:27

    SPEAKER 1 [continued]: during which people are exposed is extended,the point at which people stop getting illwill be the maximum incubation periodfrom the end of the exposure.Right, so those first two types of outbreaksare a function of environmental exposure, the point sourceand the common source.Now let's talk about what would happen if we have personto person spread, so what we call a propagated outbreak.

  • 02:48

    SPEAKER 1 [continued]: Our index case gets infected, and after a period of time,he becomes symptomatic.We've already said that between the periodof being infected and becoming symptomatic,we call that incubation period.And there's a period of time during whichthat person is infectious.In other words, they can pass it on to other people.And as you can guess, that's called the infectious period.Importantly the infectious perioddoesn't necessarily start after the person becomes symptomatic.

  • 03:10

    SPEAKER 1 [continued]: A person can actually be spreading the diseasebefore they become ill.And the period of time between becoming infected and becominginfectious is what we call the latent period.So to remember this, remember that the word latentstarts with L. Stands for lazy.During that period of time, the virus or the microbeis too lazy to spread.And the incubation period starts within.And after the incubation period you have to stay in your house

  • 03:32

    SPEAKER 1 [continued]: or in your bed.Now let's look at that exact same scenario,but in a slightly different way.Let's imagine that we've got an index case,we're going to call him John.And at some point in time, John becomesinfected with the flu virus.And at this point in time of course,John doesn't feel unwell.So he goes off to work and is surrounded by his co-workers.At this point in time, remember that John isn't ill.So he's still within the incubation period.And at this point in time, he's not infectious.

  • 03:54

    SPEAKER 1 [continued]: So it's still within the latent period.After a period of time, John does become infectious.So that's the end of the latent period.And at the end of the latent period,he's now spreading the disease.So of course, his co-workers start getting sick.And after that he, becomes symptomatic.That's the end of the incubation period.He goes home he goes to bed.But at that point, it's too late.He's already spread the flu virus on to his co-workers.Now let's take a look at what this exact same scenario looks

  • 04:15

    SPEAKER 1 [continued]: like on an epicurve.So John becomes sick.After a period of time, he'd become symptomatic.At the point at which he becomes symptomatic or ill,we see him pop up on the epicurve.But we know that John was infectiousbefore he became ill.So the point in time when his co-workers or the peopleat work, the population that wereexposed in this case, the point in time at which theywere exposed preceded the first block on the epicurve.

  • 04:38

    SPEAKER 1 [continued]: And after the minimum incubation periodfollowing the point in time in which the co-workers wereexposed, the co-workers start becoming symptomatic.And they start popping up on the curve.And more and more of John's co-workers,and maybe his family as well, willbecome infected during the entire lengthof his infectious period.That's the entire blue line.And after John's infectious period,we'll continue to see people getting sick.

  • 04:60

    SPEAKER 1 [continued]: And they'll get at a point in timethat corresponds with their respective incubation periods.And those incubation period we know,will continue until the maximum incubation period.But of course, that's not the end of the story,because those co-workers themselves, become infectious.And they become infectious before they become symptomatic.They each have an infectivity period.And so the virus begins to propagate one generationto the next.

  • 05:21

    SPEAKER 1 [continued]: And so we're seeing this entire cycle repeat itself.But as it repeats itself, we have more peoplethat are infectious.Initially, it was just one person.Now, there's a group of people.And so the respective spikes on the epicurveget higher and higher.And these peaks eventually start to coalesce or mergeinto a wave.And then eventually, the epidemicwill begin to die off and burn itself out.So why is it that these epidemicstend to burn themselves out.

  • 05:42

    SPEAKER 1 [continued]: Great question.I'm glad you asked.Here's the answer.In most outbreaks, the pathogens tendto run out of susceptible people, peoplethat can be infected.And there's a few reasons for this.Firstly, there'll be people in the population who get sickand recover, and are then immune,and are no longer susceptible to be getting sick again.Secondly, if it's a deadly disease,there may be people that die.And of course, dead people can't get sick.

  • 06:03

    SPEAKER 1 [continued]: Thirdly, as the outbreak unfolds,more and more of the population are in fact sick or infected.And those people are, of course, not candidates to be infected.And finally and importantly, theremay be public health control measures put in place,things like education, treatment,maybe even a quarantine, to fraught the onward spreadand the onward transmission of the disease.And if the disease does not peter out completely,It may become what we call endemic.

  • 06:24

    SPEAKER 1 [continued]: And that means that there's low-level, ongoing transmissionwithin a community.And that's more or less how to interpret epicurvesin a nutshell.

  • 06:31

    SPEAKER 2: Please hang up and try again.

  • 06:33

    SPEAKER 1: Thank you for watching.I hope you found that useful.

Video Info

Series Name: Gregory Martin

Episode: 7

Publisher: Gregory Martin

Publication Year: 2017

Video Type:Tutorial

Methods: Epidemiology, Graphs, Data analysis skills, Control variables, Data visualization

Keywords: data visualisation; distribution; epidemics; epidemiologic measurements; epidemiology; graphical presentation of data; infectious disease; infectious disease incubation period; infectious disease transmission; outbreak investigation ... Show More

Segment Info

Segment Num.: 1

Persons Discussed:

Events Discussed:



Greg Martin, Editor-in-Chief, Globalization and Health, discusses three examples of outbreaks—point-source, common-source, and propagated—and explains how to interpret epidemiological curves.

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How to Interpret an Epidemiological Curve

Greg Martin, Editor-in-Chief, Globalization and Health, discusses three examples of outbreaks—point-source, common-source, and propagated—and explains how to interpret epidemiological curves.

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