We had another prison outbreak here in Tennessee that hit the reports today. Someone asked:

…how does your theory account for prisons? The prison that reported 965 cases today has a capacity of 1676 but at capacity, that means 58% of the population is currently infected.

Thankfully, this one is actually an easy one — and a really good question.

It’s due to two factors: confinement, and testing protocol.

So, when I talk of “confinement” I’m not talking strictly in the “your ass is in a prison” type of thing. Rather it’s the fact that you have a large group of people in very close contact so as to maximize the risk of transmission. It’s the same reason that closing things like churches early on was a very good idea. In places like these, the actual herd immunity percentage is going to be mighty close to 100% because you’re surrounded by the virus. More on this in a bit.

The other aspect is the testing protocol of this. In the general public people who feel sick have a much higher tendency to get tested. If you have a hunch you may have been exposed you might get tested as well. The difference in prison: “line up boys, you’re going to get this swab up your nose.”

For reference, in previous prison outbreaks in Tennessee, there was around a 95% asymptomatic rate of the inmates who tested positive. Those 19 out of 20 people wouldn’t be nearly as likely to get tested if they weren’t basically ordered to get tested. I will admit that I don’t know what percentage of the inmates eventually developed symptoms, but it’s still a really big number. (This also bolsters the JAMA numbers in terms of the under testing/reporting amount in a way)

So how can herd immunity be lower in the general public than it is in prison? That part is easy, but a bit hard to put into words. It basically boils down to the circle of people you interact with on a daily basis along with the ease of transmission of the contagion. Once you have enough of the population immune it just breaks the chain of transmission. It’s not like you have the entire population in a room at once. Even if you’ve never had it, as long as the people you generally interact with have already had it then you’re protected by your friends’ immunity. (It really is more complicated with lots of stats and stuff… but it is a thing.)