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Mar 4, 2020

How is Case Fatality Rate (CFR) Calculated During an Epidemic

Posted by in category: Understanding Epidemics

Edi­tors Note: This post is reshared from Quora because it helps us under­stand how we cal­cu­late CFR (case fatal­ity rates) from epi­demics like #COVID-19. Link to the orig­i­nal post is here:

Post Author: C Stu­art Hard­wick, Award-Winning Scifi Author, Ana­log regular

Guess what? Epi­demi­ol­o­gists aren’t dumb.

Mor­tal­ity rate never includes undi­ag­nosed cases—of any dis­ease, at least not in the way you mean.

The mor­tal­ity rate is a way of com­par­ing the sever­ity of known dis­eases. Roughly speak­ing, it mea­sures the per­cent­age of known or pre­sumed cases that lead to death. Known or pre­sumed cases is a num­ber based on the con­sen­sus judg­ment of trained epi­demi­ol­o­gists. There is no other way to get it because we do not have any way to rou­tinely test even a ran­dom sam­pling of the pop­u­la­tion at large for any disease.

The Span­ish Flu of 1918 is esti­mated to have had a mor­tal­ity rate of 2.5%, but not one sin­gle diag­no­sis was made based on a lab test look­ing for viral DNA or cir­cu­lat­ing antigens—those tech­nolo­gies did not exist.

Today, they do, but they are still only one tool in the epi­demi­o­log­i­cal toolkit—for the sim­ple rea­son that such tests are nec­es­sar­ily virus-specific, and can never be made ready fast enough or in large enough num­bers to test an entire pop­u­la­tion dur­ing an emerg­ing outbreak.

Instead, known or pre­sumed cases are iden­ti­fied first by look­ing at the clin­i­cal presentation—symptoms, then by per­form­ing tests. When avail­able, the two are com­bined with sam­pling parts of the pop­u­la­tion and with esti­mat­ing dis­ease bur­den through­out the com­mu­nity as a whole.

But the num­bers are always esti­mates, and can never be any­thing else, and the meth­ods used by epi­demi­ol­o­gists are often con­founded by local con­di­tions, or even by happenstance.

As I write this, num­bers released by the Iran­ian gov­ern­ment imply a ter­ri­fy­ingly high 11% mor­tal­ity rate for SARS-CoV-2. How­ever, we have good rea­son to believe the Iran­ian gov­ern­ment is attempt­ing to hide the extent of the out­break there, and deaths are harder to hide than infec­tions. This explains the anom­alously high mor­tal­ity rate, as reported.
On the other hand, data from China tells an inter­est­ing story. As of Feb­ru­ary 20th, China is report­ing mor­tal­ity rates of 5.8% in Wuhan and only 0.7% other areas.

Does that imply the COVID-19 is not a big deal? No. Def­i­nitely not.
First, .7% mor­tal­ity is seven times worse than the aver­age for sea­sonal flu, and the novel coro­n­avirus is about twice as con­ta­gious. So going from that num­ber alone, you can expect an uncon­trolled COVID-19 out­break in the US to kill three-quarters of a mil­lion people—except con­ta­gion doesn’t scale lin­early, so the num­ber would actu­ally be higher than that.

Sec­ond, that low rate out­side Hubei province is in a pop­u­la­tion already on high alert, tak­ing sten­to­rian mea­sures to con­tain the out­break, and in which cases can be quickly iden­ti­fied and effec­tively treated, includ­ing with anti-viral med­ica­tions. The ini­tial fatal­ity rate in Wuhan was a whop­ping 17.3%! The kind of inten­sive treat­ment needed to keep mor­tal­ity rates that low just can­not be main­tained if the virus is allowed to sicken mil­lions all at the same time.

Coro­n­avirus prob­a­bly isn’t very seri­ous” is pre­cisely the WRONG con­clu­sion to draw from this data. The cor­rect con­clu­sion is, “Coro­n­avirus is very, very seri­ous, but if we can slow it down enough, we can pre­vent it from becom­ing a catastrophe.”

How do we do that?
Wash hands cor­rectly. Learn how to wear a mask prop­erly. Com­ply with con­trol orders from health offi­cials. And just in case, des­ig­nate a room in your home as an iso­lated sick ward, and give some thought to how to pro­vide care while min­i­miz­ing the spread of virus par­ti­cles through phys­i­cal con­tact and exhaled droplets.

It’s look­ing increas­ingly unlikely that this virus can be con­tained, but the more we can slow it down, the bet­ter the care we can pro­vide to those who catch it. That’s as true in our indi­vid­ual homes as it is in nations.
And if we slow it down enough, we can stop it with a vaccine.