Thread by @GidMK

Ok, so let's talk HERD IMMUNITY

This is an epidemiological concept that describes what happens when enough people are immune to a disease that it can't spread 🧵

https://twitter.com/Asher_Wolf/status/1238968686205009920

There are two essential concepts to think about with herd immunity:

The BASIC reproduction rate: R0

The EFFECTIVE reproduction rate: R

R0 describes the number of people that each infected individual will pass the disease on to when they get sick in a completely vulnerable population

For #COVID19 this number is currently estimated to be ~2.5-3t

R, on the other hand, describes the number of people that each infected individual will go on to infect given a range of different scenarios

For example, the R0 of #COVID19 is 2.5-3

However, the R of the disease was brought down in some places to just 0.3, meaning each 3 infected people passed on the disease to only 1 more person

So what does this have to do with herd immunity?

It's pretty simple. If the R of a disease is above 1, the disease will keep spreading

One way you can bring it down is with vaccines

Another is by having everyone get the disease

So, we know that if R drops below 1, the disease stops spreading

Therefore, we want to know what proportion of the people in the population need to be immune to the disease for R<1

This is the herd immunity threshold

The formula is pretty easy:

R0×%immune<1

Therefore, 1/R0 = herd immunity threshold

For #COVID19, R0=~2.5

Therefore, 1/2.5 = the % of people who need to be immune for the disease to be contained

In this case, that's 0.4, or 40%

I.e. if 40% of people get #COVID19, the disease will probably stop depressing by itself (as long add it confers lasting immunity)

Similarly, if enough people self-isolate, or if social distancing reduces the effective reproduction rate low enough, the disease may stop spreading

However, this is not herd immunity

Apologies, mistake in the number above

It is 1-40%, or 60% who need to be immune for herd immunity given this R0!

(This is why you shouldn't tweet and play Borderlands 3)

Also, good to note that herd immunity requires a LOT of people to get infected

If 60% of the population gets sick, and 1% of the cases die, that's 0.6% of THE ENTIRE POPULATION dead

In the UK, that's millions of people