Coffins of deceased people stored in a warehouse near
Bergamo — a city at the heart of Italy's coronavirus crisis — before
being transported to another region for cremation.
Piero Cruciatti/AFP via Getty Images
The coronavirus appears to be much more lethal in some countries than in others.
In
Italy, about 10% of people known to be infected have died. In Iran and
Spain, the case fatality rate is higher than 7%. But in South Korea and
the U.S. it's less than 1.5%. And in Germany, the figure is close to
0.5%.
So what gives?
The answer involves how many
people are tested, the age of an infected population and factors such as
whether the health care system is overwhelmed, scientists say.
"Case fatality rates have been very confusing," says
Dr. Steven Lawrence,
an infectious disease expert and associate professor of medicine at
Washington University School of Medicine in St. Louis. "The numbers may
look different even if the actual situation is the same."
So
it's likely that the seemingly stark difference between Germany and
Italy is misleading and will diminish as scientists get more data,
Lawrence says.
Also, because of the way countries monitor pandemics like the
coronavirus, he says, the case fatality rate tends to decrease over
time. The reason: When a new disease first shows up, testing usually
focuses on severely ill people who are at high risk of dying. Later on,
testing is more likely to include people with milder illness who are
less likely to die.
That's what happened with West Nile virus,
which appeared in the U.S. in 1999. At first, when scientists only knew
of about a few dozen cases, it appeared the mortality rate was higher
than 10%. But wider testing eventually found hundreds of thousands of
people who'd been infected but never got sick enough to notice. Today,
more than 3 million Americans have been infected and studies show that
fewer than 1% become seriously ill.
If that pattern holds for coronavirus,
countries
such as Italy, which have been testing only the sickest patients, are
likely to see their case fatality rates fall. But countries such as
Germany, which has been testing both critically ill people and those
with milder symptoms from the beginning, are less likely to see major
changes in the case fatality rate.
The U.S. is somewhere in
between. Testing was severely limited when cases started to appear.
Since then labs have begun testing tens of thousands of people with less
severe illness.
A country's case fatality rate is simply the
number of deaths (the numerator) divided by the number of infections
(the denominator). The problem is, both of these numbers may be
unreliable.
For example, when an outbreak begins and health
officials aren't looking for the virus, some people may die at home and
never be diagnosed. That would lower the numerator and "might lead to an
underestimate of the case fatality rate," Lawrence says.
But a
much more likely scenario, he says, is that early in an outbreak,
testing is limited to people who are so sick they wind up in the
hospital. That means the only infections that get counted are in the
people most likely to die. So the denominator is missing a huge number
of infected people who survive, and that makes the virus appear much
more deadly than it really is.
This is probably one reason that early death rates in China appeared so high, says
Gerardo Chowell,
a professor of epidemiology and biostatistics in the department of
population health sciences at Georgia State University. Chowell is part
of a team that has been using statistical modeling to study the outbreak
in China and South Korea.
When cases started showing up in the
city of Wuhan, Chinese health officials "were obviously caught by
surprise" and lacked the ability to test many patients, Chowell says. So
testing was restricted to the sickest people. That probably contributed
to early evidence that the fatality rate in Wuhan was 4% or more.
A
study
published last week estimated that in Wuhan, the chance that someone
who developed coronavirus symptoms would die was actually 1.4%.
In
South Korea, though, "they have been doing massive testing" since the
first cases were detected, Chowell says. As a result, that nation has
been able to count infected people with mild symptoms as well as those
who become severely ill. That may be one reason the case fatality rate
in South Korea has remained below 2%.
Another factor affecting
coronavirus fatality rates is the characteristics of the population that
is infected at any given moment, says
Mary Bushman, a postdoctoral researcher at Harvard's Center for Communicable Disease Dynamics and an author of the Wuhan study.
In
Washington state, Bushman says, the first cases appeared in nursing
home residents, who tend to be extremely vulnerable to the disease. That
produced "an alarming number of deaths being reported," Bushman says.
At one nursing home, 34 of 81 infected residents died, which is a case
fatality rate of 42%.
But as Washington began testing for the
virus outside the nursing home, it became clear the case fatality rate
in the general population was vastly lower.
And across the
U.S., as testing has expanded to include younger and healthier segments
of the population, the fatality rate has decreased to levels similar to
those in South Korea. "And I think we'll probably continue to see
further decreases," Bushman says.
Differences in testing
aren't the only reason that case fatality rates vary, though. In some
countries, infected people have been more likely to die because the
health care system has been overwhelmed, leaving critically ill
coronavirus patients without access to lifesaving care, Chowell says.
In
Wuhan, he says, high case fatality rates early on were probably caused
in part by the inability of local hospitals to handle the huge influx of
patients sick with the coronavirus.
An overburdened health
care system may also be contributing to the high case fatality rate in
Italy. "During those high peaks where the health care systems can be
overwhelmed, there may not be enough people or ICU beds or ventilators
to be able to provide the critical care that is needed," Lawrence says.
Ultimately, it will take a different sort of test to assess how
lethal coronavirus has been, Lawrence says. Most current tests only
detect active infections — when the virus is still present in the body.
But a different type of test — now being developed but still probably
months away from wide use — can reveal whether a person has ever been
infected. And that is what scientists need to know to establish the true
denominator for coronavirus and to find the true case fatality rate.
In
the U.S., it's likely that the case fatality rate from coronavirus will
end up somewhere between 0.5% and 1%, once a broad cross-section of the
population has been tested, Lawrence says.
But that's no reason for the nation to relax, he adds.
"To
put it into perspective, that's 5 to 10 times more fatal than flu,"
Lawrence says, a disease that kills between 12,000 and 61,000 people a
year.
The modelling showed that implementing measures early on can have a dramatic impact.
