Hello, in this video we'll tackle one of
my favorite topics, the epidemic curve.
That's a thing you probably
see a lot of on TV or in textbooks,
the incidence rate going up
and down over time.
So we'll go over some of the
different kinds of epidemic curves
and how we relate them to
the different kinds of epidemics.
I hope you'll learn something and I
hope you enjoy the content, thank you.
An epidemic curve shows the
frequency of new cases over time.
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.
The shape of the curve in relation to the
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.
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.
So right away, just by looking at the
shapes, we can know a lot about the nature
of the outbreak.
Things that "Epi curves"can teach us include
the time trend of the outbreak so
the distribution of cases over time.
it can show us who the outliers are.
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.
Those will be unusual occurrences.
It also would give us a general
sense of the outbreak's magnitude.
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.
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.
So if you remember our different kinds of
outbreaks, we have a point-source epidemic.
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.
So a point-source outbreak
tends to look like this.
It's a single wave.
So its cases increase over time and
then they decrease a little bit more slowly.
And all the cases tend to fall within
one incubation period of the pathogen.
So it's fairly easy to identify.
A continuous common source epidemic,
however, rises to a peak and then falls.
So in this curve you can see it goes up and
it goes down but it stays up a little bit longer.
That's because the
infection is lasting a bit longer.
An example would be tainted water.
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.
And eventually, the infections go away
as we discover the source of the infection
and close off that water source as it is.
So again, the shape here implies there's
an ongoing source of contamination.
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.
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.
And when it dumps water into the river, it
isn't for the same amount of time all the time.
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.
So the onset is abrupt and the cases
are spread over a great period of time.
The incubation period will depend
on how long the exposure lasts,
how long that source is
producing toxin or infection.
A propagated epidemic is one
that moves from person to person,
like an infectious disease
or respiratory disease.
and cases occur over more
than one incubation period.
so they usually look like a series of waves.
That's why things like the flu
or COVID-19 happen in waves.
It's not uncommon for the waves,
the curves to be successively larger
before coming down again.