So let's go back to the
CFR, the case fatality rate,
which again, is the proportion of
known cases that end up dying.
Several things affect the CFR.
Here are a few of them.
First is the clinical experience that
doctors have in treating the disease.
So the more experience we have, the
more likely we are to be able to prevent death.
The more complicated and high technology
and well resourced the health care system,
the better able we are to fend off death.
So it's not unreasonable to assume that
wealthier countries with better healthcare systems
might have lower CFRs for the
same disease than poor countries with
lesser resourced healthcare systems.
And as an epidemic unfolds throughout time,
then we get more clinical experience and
the CFR probably will come down as well.
Now that might change if
the epidemic is so severe
that the health care system becomes
overwhelmed, doctors become exhausted,
the resources get used up.
In such a case, even though more time has
passed and more clinical experience has been gained,
the CFR might go up because the
healthcare system is unable to respond as well.
Of course, the patient always provides
the most variants in these analyses.
So comorbidities affect CFR.
In the case of COVID-19, we know that
people with diabetes or hypertension or obesity
are more likely to die than
people without these comorbidities.
So population with more people with
comorbidities is going to have a higher CFR
than populations without those comorbidities.
One could also consider age to
be something of a comorbidity.
So, a population that skews older might
have a higher CFR than one that skews younger
for diseases like COVID-19.
And lastly, new treatments
emerge as an outbreak unfolds
and more resources are
put to solving the problem.
So, in the case of COVID-19, the
advents in using monoclonal antibodies
or some of the new antiviral drugs
came months after the first cases.
As a result, the probability of dying if you
become infected comes down somewhat
if you have access to some
of these new medications.
So the CFR is not a biological constant.
It's entirely dependent
on the context of infection.
And by context, I mean where you got
infected, when you got infected, who you are
and what you have access
to in terms of treatment.
Other issues with the CFR include that it really
does depend on how we define cause of death.
It isn't always clear whether the
disease was the likely cause of death
because most deaths are
the result of many factors.
So some clinical determination, a
qualitative determination and opinion
must be garnered by a clinician
to determine whether or not a certain
death was likely caused by the disease.
As well, prolonged sickness that does
not lead to death might complicate matters.
So what do I mean by that?
At best, we hope that when
computing the CFR, the people who
got infected and the people who died did
so within somewhat of the same timeframe.
But individuals who gets sick and
linger for a very long period of time,
weeks, months and sometimes years
and then die complicate the data somewhat
because their infection was
recorded an earlier time period
and their death recorded in a
later time period.
So that might skew the CFR
computation to be less severe-looking
if an individual has not
died yet, but will die later on.
Of course, measuring the
denominator is a big issue.
The denominator again is the
number of people with the disease.
So consider an overburdened
health care system
or the early days of the COVID-19
epidemic in well-resourced countries
in Western Europe or North America.
In general, there are so many cases and so
few tests and the system was being overwhelmed
that only the most symptomatic cases
were presenting themselves to be tested.
As a result, you find cases that are
more serious that are more likely to die.
So it makes sense in such a scenario
for the CFR to be higher in the earlier days.
And as more testing becomes available,
and we test more asymptomatic
or lesser symptomatic people,
you get lesser severe disease and
individuals who are less likely to die.
So the CFR should come down.
As a result, CFR will change over time, even
in the same population in the same country,
depending on the nature of testing.