Know how to interpret an epidemic curve?

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 how to 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 talk about how to interpret them how to make sense of them and of course I'm going to revise briefly what epidemiological curves are hello and welcome back to this global health youtube channel my name is Greg Martin I just want to quickly send a big thank you to the University of Notre Dame for sponsoring and supporting this video I couldn't do it without you thanks very much for those of you that are watching if you're interested in studying global health and getting a good sense of the science that underpins global health practice the University of Notre Dame run a Masters of Science in global health program that is absolutely outstanding I'll talk a little bit more about it at the end of the video so firstly what is an epic curve or an epidemic curve an epidemic curve is a visual representation of 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 outbreak and that's when a population or a group of people are exposed to a pathogen at a single source 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 wedding and there's some dodgy food and they all get sick a few days later and the second is a common source outbreak and this is when people are exposed to some sort of environmental hazard that takes place over a period of time like if there's pathogens in a water source and finally we're going to talk about propagated outbreaks and that happens when there's an infectious agent that spreads 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 exposed at the same point in time they all attended the same wedding they all ate the same food not everybody got sick at the same time the reason for this is that there's a 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 people being exposed to the pathogen and actually becoming ill is called the incubation period for each infectious agent we know that there's a period of time during which nobody gets ill 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 distribution of incubation periods now let's imagine that this wasn't a point-source outbreak let's imagine that this was a common source heartbreak and basically that means that the population 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 exposed for an extended period of time and remember that in a point-source outbreak the people continue to get sick until the end of the maximum incubation period so in a common source outbreak because the period of time during which people are exposed is extended the point at which people stop getting ill will be the maximum incubation period from the end of the exposure right so those first two types of outbreaks are a function of environmental exposure the point source and the common source now let's talk about what would happen if we have person-to-person spread so what we call a propagated outbreak our index case gets infected and after a period of time he becomes symptomatic we've already said that between the period of being infected and becoming symptomatic we call that the incubation period and there's a period of time during which that 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 period doesn't necessarily start after the person becomes symptomatic a person can actually be spreading the disease before they become ill and the period of time between becoming infected and becoming infectious is what we call the latent period so to remember this remember that the word latent starts with L L stands for lazy during that period of time at the virus or the microbe is too lazy to spread an incubation period starts within and after the incubation period you have to stay in your house or in your bed 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 becomes infected with a flu virus and at this point in time of course John doesn't feel unwell so he goes off to work and he's 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 so it's still within the latent period after a period of time John Dez becoming fix's so that's the end of a latent period and at the end of the latent period he is 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 coworkers now let's take a look at what this exact same scenario looks like on an AP curve so John becomes sick after a period of time he becomes symptomatic at the point at which he becomes symptomatic or ill we see him pop up on the AP curve but we know that John was infectious before he became ill so the point in time when his co-workers or the people at work the people at the population that were exposed in this case the point in time at which they were exposed preceded the first block on the AP curve and after the minimum incubation period following the point in time in which the co-workers were exposed the co-workers start becoming symptomatic and they start popping up on the epic curve and more and more of John's co-workers and maybe his family as well will become infected during the entire length of his infectious period that's the entire blue line and after John's infectious period will continue to see people getting sick and they'll get sick at a point in time that corresponds with their respective incubation periods and those incubation periods 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 infective 'ti period and so the virus begins to propagate one generation to the next and so we seeing this entire cycle repeat itself but as it repeats itself we have more people that are infectious initially it was just one person now there's a group of people and so the respective spikes on the EP curve get higher and higher and these Peaks eventually start to coalesce or merge into a wave and then eventually the epidemic will begin to die off and burn itself out so why is it that these epidemics tend to burn themselves out great question I'm glad you asked he has the answer in most outbreaks the pathogens tend to run out of susceptible people people that can be infected and there's a few reasons for this firstly there'll be people in the population who gets sick and 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 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 there may be public of control measures put in place things like education treatment and maybe even quarantine to float the onward spread and the onward transmission of the disease and if the disease does not peter out completely it may become what we call endemic and that means that there's low-level ongoing transmission within a community and that's more or less how to interpret AP curves in a nutshell thank you for watching I hope you found that useful remember to subscribe to this channel if you haven't already now don't go anywhere stay for one minute I want to tell you a little bit more about the University of Notre Dame's Master of Science in global health program for those of us that are already working in the global health space or in the public health space you and I both know that from time to time people ask us about what they should study where they should study and in that sense I want you to think about learning more about the University of Notre Dame's Master of Science and global health program it produces graduates that have an excellent understanding of the science that underpins the practice of global health thanks for watching see you [Music]

Channel: Global Health with Greg Martin

Views: 38,601

Rating: 4.9365077 out of 5

Keywords: epidemic curve, epidemic curves, epidemiology, epidemiologist, infectious disease, outbreak, epidemic, pandemic, public health, global health

Id: 7SM4PN7Yg1s

Length: 7min 38sec (458 seconds)

Published: Fri Jan 06 2017