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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Health Nerd Retweeted Health Nerd
11/ But that brings us back to this earlier point The only way, currently, to be immune to the disease is to GET SICK and then GET BETTERhttps://twitter.com/GidMK/status/1247328596890275840?s=20 …
Health Nerd added,
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12/ So what we're really saying is that, for the herd immunity threshold to be reached, ~67% OF THE POPULATION has to be infected That is a HUGE problem
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13/ How much of a problem? Well, assuming a relatively generous infection-fatality rate of 0.3%, if 67% of Australians get infected with
#COVID19 then ~50,000 people die A high price to pay indeed2 replies 1 retweet 13 likesShow this thread
14/ Taking more realistic estimates of the fatality rate, we might see 100,000 or more people die before reaching this point Most would, I think, regard that as the most abject of failures
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15/ By definition, herd immunity is not and can never be a solution to
#COVID19, unless we are comfortable with a staggering death toll fin5 replies 5 retweets 33 likesShow this thread -
16/ IMPORTANT ADDENDUM: "never" is the wrong word in the previous tweet. If we get a vaccine, or if the death toll drops significantly (due perhaps to better treatments) herd immunity becomes a very viable option
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