So, yet again we have a wild misrepresentation of "herd immunity" as a solution to the coronavirus pandemic
This is simply not the case. If herd immunity comes into play, we will have already seen one of our worst-case scenarios played out in real life 1/n
This is simply not the case. If herd immunity comes into play, we will have already seen one of our worst-case scenarios played out in real life 1/n
2/ Herd immunity is a simple concept
It simply defines the situation where enough people are immune to a disease that even if people do get sick, they can't spread the infection
It simply defines the situation where enough people are immune to a disease that even if people do get sick, they can't spread the infection
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
theguardian.com/society/ng-int…
If enough people are immune, the disease spreads a bit, then stops
theguardian.com/society/ng-int…
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 3
For measles, this number is 18
Influenza is roughly 1.5
For #COVID19, R0 is around 3
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
There's a simple mathematical equation to work out by how much:
R = R0*% susceptible
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
Therefore, the R has to drop below 1
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!
That carries very serious risks!
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
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
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
We want R to be below 1
This makes it into a simple equation: R<1
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
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
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
This is what's known as the HERD IMMUNITY THRESHOLD
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 BETTER
The only way, currently, to be immune to the disease is to GET SICK and then GET BETTER
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
That is a HUGE problem
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 indeed
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 indeed
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
Most would, I think, regard that as the most abject of failures
15/ By definition, herd immunity is not and can never be a solution to #COVID19, unless we are comfortable with a staggering death toll
fin
fin
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
