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Herd Immunity for dummies

April 21, 2020

Humans, 

Let’s talk about herd immunity. There has been and will continue to be a lot of conversation about herd immunity in the weeks to months to come so I thought I would break it down. 

We have all seen the animal planet where the lion chases the antelope herd and takes out the weak one and the rest of the herd is spared. That is a great model for not getting eaten by a lion but it really doesn’t have much to do with herd immunity and viruses.  

Herd immunity is an epidemiologic concept that describes when a population is no longer susceptible to a virus spreading at will. Meaning that the remaining people who are susceptible will no longer be at increased risk for getting the virus. Recall a virus can only replicate in a susceptible host. That fact is key. 

So let’s come up with some extreme examples to illustrate herd immunity. Imagine a room with 1,000 people - it’s a big room. In this fictitious big room, there is one person infected with FICTITION and only one person susceptible to FICTITION. The rest of the 998 people are immune to FICTITION (they can’t get it OR give it). In order to get/give FICTITION a person must shake hands and talk continuously for 2 minutes with another person. 

Back to our room - the 1000 people are placed randomly throughout the room, shaking hands and talking to others for 2 min at a time. This event lasts for 2 hours. 

You can imagine the likelihood that the FICTITION infected person will give FICTITION to the susceptible person in that time frame is very low. In fact, depending on how the room is laid out and how talkative people are, the susceptible person may never get FICTITION even if you removed the time limit. That susceptible person has effectively become immune because of the 998 immune people… hence the term herd immunity. This is basically measles. 

You can also imagine the inverse scenario where 998 people are susceptible, 1 person is immune, and 1 person has active FICTITION. The illness would spread much easier. This is basically COVID. 

Now let’s play with the variables…

  • Imagine if 800 people are immune, 190 are susceptible, and 10 people have the infection. Is the group of 190 susceptible people as protected as the prior example when there was only one susceptible person? Of course not. 

  • Imagine if you could transmit the disease with just a handshake and no conversation (more contagious). Would there be more cases of FICTITION at the end of 2 hours? Of course, there would be.

  • Imagine the opposite, you need to hug and have a 10 min conversation in order to get/give FICTITION (less contagious). Would there be less FICTITION in 2 hours? Of course, there would be.

  • Now imagine if you put in a constraint that you can only talk to one person every 1 hr (social distancing). How would FICTITION spread in this example? Yep, much slower.

Here’s the deal, for complicated mathematical reasons that I do not understand, depending on how contagious something is, you need between 70-90% of a population to be immune for there to be herd immunity. There are only two ways to achieve immunity either people need to contract the infection themselves or be conferred immunity from a vaccine. 

There are a lot of topics to riff on at this point that I will pick off over the coming days. 

  • COVID mortality

  • Testing for past infection 

  • The concept of R0 

  • The value of a human life

  • The negative medical consequences of economic hardship

My brain hurts and long letters are not as much fun to read. But, if you need more on this topic, here and here are more scientificy (yes, I made that word up) articles I found helpful. Here is a more newsy one with a great image. This is a good visual too. 

I will end today’s email with this. I saw my first COVID patient that needed to be hospitalized yesterday. He left Against Medical Advice despite my best efforts. He planned on going to the pharmacy and the vet clinic after he left my ER and clearly wasn’t social distancing. If you don’t think you need to be vigilant when you’re out there, think again. 

Stay emotionally connected and physically distant,

Greg

PS: Follow up on the ventilators letter - someone sent me this cool video that describes some of the reasons why good ones are hard to make.