April 30, 2020
Humans,
Before we talk about mortality, let’s revisit the three topics already addressed in the “For Dummies” series. They are building on each other and these topics are critical to understanding the science of easing social distancing (the Dance). So far we covered:
Herd Immunity - we learned herd immunity refers to the concept that once we reach a certain threshold of immunity (prior infection or vaccine) in the population then the virus will have a very hard time spreading. With COVID, the best guess is that it will take 60-80% of the population to be immune in order to achieve herd immunity.
R_{0} - we learned that R_{0} is a numerical way to communicate contagiousness (COVID’s is likely around 2 - 5.7, Spanish Flu was 1.4 - 2.8, and Measles is 12 - 18). We learned about a variation of R_{0} called R_{T}. This communicates a real-time estimate of the contagiousness of the virus in a given place at a certain time. I directed you to a site that reports on real R_{T} values for each state.
Prevalence - we learned that this is a % that shows the number of people that have or have had the illness. This is a very hard number to pinpoint right now because with a low prevalence disease you need an almost 100% accurate test to trust the result of the test. But, our best evidence is that in the US we are likely around 2-5% prevalence (as of late April).
So, I hope you can see I’m trying to build your knowledge so you will have a basic understanding of the scientific concepts that will influence the Dance. I’m sure there are a lot of ways to go about the Dance, many states will do this differently. We are in a huge science experiment. But, recall as a population we all should have the goal of preventing uncontrolled exponential growth that will overwhelm the healthcare system. I continue to use the terms of the Hammer and the Dance and will continue to use these terms as they are a perfect way to conceptualize what we as Humans will need to do. If you haven’t read the Hammer and the Dance article please do so.
Some definitions:
Mortality (a term for death) vs Morbidity (a term for negative effects of an illness). I will only be talking about mortality (or death) here. We are finding this virus negatively impacts almost every organ system, including causing strokes in the young with mild disease. There is plenty of morbidity, but I’m not addressing this in this email because we don’t know enough at this time and it will make this letter way too long. Certainly, morbidity is super important to consider once we get into the cost-benefit equation of limiting COVID disease vs opening economy (spoiler alert - there are no good answers here - I certainly don’t know what the right answer is and anyone that says they know the answer is a liar - I am just helping to explain the science and give you information so that you can make personal decisions for you, your family, and your friends).
Mortality rate - the number of deaths in a given population over a given time. This estimates the risk of dying of a certain disease over a certain time. This speaks to the chance of dying from an illness in a given population.
mortality rate = number of deaths / the size of the population
Case fatality rate (or case fatality ratio) - the proportion of people who die from a specified disease among all individuals diagnosed with the disease. This estimates the risk of dying if you get the disease. This speaks to the severity of a particular illness.
Case fatality rate = number of deaths / number of individuals diagnosed
I found this page to be very helpful. It describes how the mortality rate and the case fatality rate can go up and/or down independently of each other. Here was their description:
As an example, consider two populations. One population consists of 1,000 people; 300 of these people have the specified disease, 100 of whom die from the disease. In this case, the mortality rate for the disease is 100 ÷ 1,000 = 0.1, or 10 percent. The case fatality rate is 100 ÷ 300 = 0.33, or 33 percent. The second population also has 1,000 people; 50 people have the disease and 40 die from it. Here the mortality rate is 40 ÷ 1,000 = 0.04, or 4 percent; the case fatality rate, however, is 40 ÷ 50 = 0.8, or 80 percent. The incidence of death from the disease is higher in the first population, but the severity of disease is greater in the second.
Without knowing the number of asymptomatic cases (or the true prevalence) of COVID, we really don’t know what the mortality rate or the case fatality rate is. Read here and here for some deep dives that explain how difficult this problem is to figure out. Additionally, there is evidence (and here) that the number of deaths attributed to COVID is being under reported.
If we ultimately find that there are a lot more asymptomatic cases than we initially thought, then the case fatality rate goes way down (as the denominator in that equation goes up) and this disease is less risky than previously thought. If we find out that we were right and the mild/symptomatic cases are about 80% of the total, then the case fatality rate will likely still drop but by much less.
Let us all hope and pray that there are WAY more asymptomatic cases than previously reported AND that previous infection confers some immunity. There are a lot of studies going on right now that are attempting to elucidate the prevalence of this disease. I discussed some of the first studies back in the “Prevalence for dummies” email.
Ok, I have told you what we don’t know… the real mortality or the real case fatality rate. Let me tell you what we do know. This disease kills the old and sick at much higher rates. This is certainly not news to anyone.
It is VERY unlikely that these two facts will change. If and when we develop better treatments and/or a vaccine, then the case fatality rate will drop but this will only affect people who get the disease AFTER these events. I know that is obvious but that is the reason you want to get COVID as far from today as possible.
Here is the only conclusion I can say with certainty. If you are in a group that has a higher case fatality rate, it is vital that you get the virus after we develop better treatments or a vaccine.
Ok, my brain hurts.
I’m going to continue to build on all the scientific concepts needed to understand the Dance in the “for dummies” series. We still have a few topics left...
Your mortality with COVID.
How many people could die from COVID?
Vaccines
The negative mental and medical consequences of economic hardship
The value of a human life
Until then…
Stay emotionally connected and physically distant,
Greg
PS: Dr. David Schriger, one of the docs from EMRAP (the ER doc educational organization I have referenced a bunch) must have heard about my “for dummies” letter series. They did a 20 min lecture/conversation covering R0 and RT and other topics. He did a much better job than I did. If you still don’t understand these concepts, please watch this. He argues that both the “keep strict social distancing” and the “end social distancing now” camps are both correct. He then also riffs on a bunch of other topics like the US healthcare system vs Swedish healthcare system, future pandemics, the future of COVID in the US, civil liberties etc. I found it quite insightful.
PPS: I have heard of the COVID pandemic described as a war. It's only appropriate that I write about mortality on the day when the COVID deaths surpass 60k. This happens to be the approximate deaths from the Vietnam War. While it took 20 years for that many US soldiers to die in Vietnam, it only took COVID about 2 months. The next regrettable milestone will be if we exceed the death toll of US troops in WWI at 113,000.
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