April 24, 2020
Humans,
I mentioned in my “Herd Immunity for dummies” email that I would be writing a few letters that would build on themselves. These topics are too long to digest in one sitting. So, if you haven’t read the Herd Immunity email, go back and read that first.
I think the next topic to understand is the basic reproduction number, R_{0}, pronounced R “naught” or R “zero.” Why? Because it allows us to discuss and understand how contagious COVID is. My goal is to help you understand this concept as well as to explain why it is very hard to pinpoint an exact number, why the number is changing, and why it can be different for different places.
I touched on the concept of R_{0} in the email where I discussed the contagiousness of FICTITION. Recall, I compared how effectively and likely FICTITION can be spread with 3 scenarios: with a handshake, with a handshake and a 2 min conversation, or with a handshake and 10 min conversation. Intuitively, everyone answered that if it is just a handshake, FICTITION would spread very quickly, and if it was a handshake and a 10 min conversation, then it would spread much slower. This, in essence, is the basic premise of R_{0}.
In the interest of keeping this topic as digestible as possible, I’m going to simplify a much larger body of knowledge to its essence. So, I’m combining the terms basic reproduction number (R_{0}) and the effective reproduction number R_{t} (or R_{e}). The only thing I want to point out is:
R_{0} is the theoretical potential reproduction number of a virus in a completely susceptible population that does not alter its behavior.
R_{t} is the actual (or effective) reproduction number for a given population at a given time.
To make it even more simple, think of R_{0} as a way to communicate how contagious something is. The higher the number the more contagious and the lower the number the less contagious. Let’s go through some basic concepts that add depth to your understanding of R_{0}.
R0 is defined as the number of other people on average that a person with the infection will infect. So, in the FICTITION example:
the scenario with just a handshake would have a very high R_{0} because it is very easy for it to spread
the scenario with a handshake and 10 min conversation would have a much lower R_{0} because it will be harder for it to spread
There are a lot of things about the characteristics of a virus that can affect the R_{0}. For instance:
Can the virus just float in the air? Does it need a droplet or does it need direct contact or blood exposure?
Can the virus infect someone with just one copy or does it need 10k or 100k copies?
Can the virus live outside a host for 1 min or 1 day or 1 year?
You can start to see that the internal characteristics of the virus will affect its R_{0}. The tricky thing is that the R_{0} is also affected by external characteristics of the population in which the virus is spreading. For instance:
Do people engage in actions that promote the spread?
Do people engage in actions that prevent the spread?
Do people even know they have the virus?
If the virus’ spread is affected by the host’s health, what is the health profile of the population the virus is spreading in?
If the virus’ spread is affected by the host’s age, what is the age of the population it is spreading in?
Ok, so you now understand that both internal characteristics of the virus and external characteristics of the population can influence the R_{0}. So, the R_{0} of a virus will have a range, and the exact number will depend on a bunch of things. Understanding that R_{0} has a range will allow you the mental flexibility to appreciate that the “contagiousness” of a virus will be different in different populations over different times. I believe that is the important piece of information to let sink in.
Now that we have a basic understanding of R_{0}, let’s talk about why the value of 1 is so important when discussing R_{0}. If for every person that has the illness more than one person will get the illness, then the illness is spreading. Conversely, if for every person that has the illness less than one person will get the illness, then the infection will no longer spread and eventually die out.
So, now with a basic appreciation of R_{0}, or really R_{t}, this website becomes really cool. It is showing the effective reproduction rate of each state. Go check out your state. Click on what it looked like yesterday, last week, etc. Please go check it out.
OK, again my brain hurts. So we have covered Herd Immunity and now R_{0}. If you want to learn more on R_{0}, read this one first, then this one, then this one, and finally this one.
Before we can even begin to answer the $64,000 question, “When will things get back to normal?” there are a few more topics we need to explore. So look for more of these “for dummies” letters. In days to come, I will cover:
Mortality rate
Testing for current and past infection
Vaccines
The negative medical consequences of economic hardship
The value of a human life
Unfortunately, it will become clear that THE question is almost impossible to answer right now which is causing such a confusing narrative.
Stay emotionally connected and physically distant,
Greg
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