3/ The Guardian produced a great model of how herd immunity works here If enough people are immune, the disease spreads a bit, then stops https://www.theguardian.com/society/ng-interactive/2015/feb/05/-sp-watch-how-measles-outbreak-spreads-when-kids-get-vaccinated …
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4/ Herd immunity is a function of the basic reproduction number of a disease (R0). This is the average number of people each sick person infects in a totally vulnerable population For measles, this number is 18 Influenza is roughly 1.5 For
#COVID19, R0 is around 31 reply 2 retweets 13 likesShow this thread -
5/ As the population gets immune to a disease, the effective reproduction number (R) starts to drop There's a simple mathematical equation to work out by how much: R = R0*% susceptible
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6/ For a disease to stop spreading, each person has to - on average - infect fewer than 1 person Therefore, the R has to drop below 1
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6.5/ And remember, there's NO VACCINE FOR COVID19 (yet), so all immunity is conferred by getting an infection and recovering That carries very serious risks!
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7/ So let's consider what would be required for R to drop below 1 in this context R0 = 3 If 10% of the population is immune to COVID-19, this will be reduced to R = 3*(1-0.1) = 2.7 The disease still spreads
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8/ Luckily, this is just maths, so we can work it out We want R to be below 1 This makes it into a simple equation: R<1
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9/ Substituting in for R, we get: R<1 => R0*(% susceptible)<1 => %susceptible<1/R0 Solving for R0 = 3, we get %susceptible<33.3% In other words, as long as more than 33% of the population is susceptible, the disease spreads
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10/ To put it another way, we need 67% (2/3) of the population to be IMMUNE to the disease for it to stop spreading This is what's known as the HERD IMMUNITY THRESHOLD
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Replying to @GidMK
So if we have a CFR we can calculate how many people need to die to accomplish this
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I give a rough estimate further down, but yes pretty much. It's somewhere between 0.2-0.7% of the entire population depending on the 'true' IFR
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Replying to @GidMK
Ha yeah I thought that might be where you were going with this
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