In this video we'll look at some
of the indicators for understanding
the propagation of infectious disease epidemics.
In particular, the CFR - the case fatality rate.
That's the proportion of people
who get the disease who die.
The infection fatality rate, the IFR,
that's the proportion of all the people
who get the disease who die,
not just the ones we know about.
See the difference there?
And lastly we'll look at the attack
rate which is one of the most important
ways we have of measuring the
impact, the progression and the intensity
of an infectious disease outbreak.
The first one is the case fatality rate or CFR
and it's one of the most
important indicators that we use.
We're measuring a lethality
especially as lethality changes over time.
Now it's important to note that the CFR,
the case fatality rate isn't actually a true rate,
or even a ratio.
It's actually a proportion of incidents, so
proportion of the incidence
rate that results in people dying,
and we usually give it as a
percentage so we compute
the proportion and multiply it by 100.
It's very simple.
It's just the number of deaths
divided by the number of cases.
So the fraction of known confirmed
cases of the disease that results in death.
And it's important to note that the
deaths are assumed
to have been caused by the disease.
And that's a bit of a controversy of course,
because medically it's
sometimes difficult to determine
whether the deaths were indeed
caused by this particular infection.
Now, while the CFR the case fatality rate
is what we compute on a day to day basis,
based upon the information in front of us, it's
really an estimate of a more important indicator,
which is the IFR or the infection fatality rate.
The infection fatality rate is the proportion of all
infections caused by the disease who end up dying.
Let's go over that again.
The CFR is the fraction of known cases that
die and the IFR is the fraction of all cases that die.
Of course, the IFR is never really truly known
because it's impossible to know all of the cases.
You only end up knowing the cases
that present themselves for testing,
the cases who are symptomatic and who have
access to the health care system to be tested.
And you never really know of all the
deaths because some people die at home
never having encountered the health care system.
So the IFR is a theoretical construct really,
and we never really get a good sense
of it until the epidemic is actually over,
when it's run its course and pretty much
most of the cases have been investigated
For COVID-19, we use the
antibody tests at the population level
to get a true sense of the denominator
i.e. the number of actual cases of infection
to get us a better sense of the IFR.
IFR just like the CFR is
expressed as a percentage.
So for some historical context, let's
look at the IFR as for the 1957 USA flu,
and the IFR for the 1918 Spanish Flu.
We think in 1957, that strain of influenza
had an infection fatality rate of about 0.27%.
Again, these are estimates based on models.
And the Spanish flu, we think was
over 2%, which is a very high number.
Right now, as of this date, for COVID-19, we
think the IFR is probably between 0.5 and 1%
but that's subject to change depending
upon new modeling, new information, new data.
The IFR comparisons give us a good
sense of again, how deadly a disease is
but also how to quantify it and
compare it to other epidemics.