# Herd Immunity

by Raywat Deonandan, PhD

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00:01 Now, let's talk about Herd Immunity.

00:04 Herd Immunity is the idea where if enough people are infected or rather immune in a population, they're able to protect the people who are not yet immune because the disease isn't able to access those people who are not yet immune for herd that protect them.

00:23 In other words, when it's a significant proportion of the population is immune, then we have herd immunity.

00:33 We tend to get herd immunity from vaccination.

00:37 But it's possible to have a component of herd immunity being offered by natural immunity recovering from a disease.

00:46 The herd immunity threshold is the percentage of the population that needs to be immune, in order for there to be herd immunity.

00:55 And to compute that number, we need the base reproduction number, the R nought.

01:01 That's given by this formula here.

01:03 The herd immunity threshold is R0 minus 1 divided by R0.

01:07 Or if you rearrange the numbers, you also get 1 over R0.

01:13 So, in the case of COVID-19, given what we observed earlier in China, in terms of what we think the R0 is, we think 50 to 60% of the population needs to be immune before the entire population is immune.

01:29 But that's probably an underestimate, as more data comes in, that number is going higher.

01:35 So again, all of these estimates are based on the quality of the existing information, which is always subject to change.

01:47 So, you can have a lower threshold of herd immunity, if we have lesser contact rates.

01:55 Let's say you can only vaccinate 40 or 50% of the population, you won't reach herd immunity, but you can artificially reach it, if you intervene with non-pharmaceutical interventions in a population.

02:10 Mask wearing, or social distancing, or preventing people from gathering more so then you artificially lower the reproduction number, and therefore, you artificially lower the herd immunity threshold.

02:25 And you do that until you're able to immunize more people and then you can lift these restrictions because you will achieve a higher herd immunity threshold.

02:35 Here are some examples of herd immunity thresholds for well known diseases.

02:39 So, measles, which is transmitted mostly by airborne pathway has a very high R0, extremely high.

02:48 12 to 18 people can be infected by an existing infected individual very quickly.

02:54 Therefore, we need a high herd immunity threshold.

02:57 Up to 95% of people need to be inoculated against measles for there to be herd effects observed at the population level.

03:06 That's why for measles, it doesn't take a lot for there to be a crisis in a population.

03:13 So, with the anti-vaccination movement, gaining ground in some countries, and some children not getting their obligatory measles vaccinations, it doesn't take a large number of such individuals to cause outbreaks to actually happen in the population.

03:31 We cannot tolerate more than 5% of people not getting inoculated by measles.

03:37 Smallpox, we thought had an R0 of between 5 to 7.

03:42 Therefore, we needed a pretty high herd immunity threshold about 86% of the high end.

03:48 We achieve that number through vigorous global inoculation.

03:54 And that enabled us to eradicate smallpox from the face of the earth.

03:58 That was due entirely to inoculation, reaching the computed herd immunity levels.

04:07 Ebola need possibly up to 60% of the population to be immune.

04:12 Influenza. Every year in many countries, there is a problem with the influenza flu vaccine uptake.

04:23 And in countries like Canada and the USA, we're lucky if we get 40% of the population accepting the vaccine.

04:31 It's lucky then that we need about that same level to get herd effects each year when the flu does its global dance.

04:42 SARS was highly infectious.

04:46 An R0 of somewhere between 2 and 5.

04:48 Therefore, a very high herd immunity threshold would be required if that disease had not gone away on its own.

04:54 For COVID-19.

04:56 We think given what we've observed about the transmissability of a disease to this point, we'll need above the 70% of a given population to be inoculated before we achieve herd immunity.

05:13 If a mutation occurs and the R0 goes up, then the herd immunity threshold will also go up.

05:20 So, the target for vaccination will increase.

The lecture Herd Immunity by Raywat Deonandan, PhD is from the course Pandemics.

1. 92 to 95%
2. 10 to 29%
3. 30 to 40%
4. 40 to 59%
5. 60 to 80%
1. R0-1/R0
2. R0 + 1/R0
3. R0 x 2/R0
4. Re - 1/R0
5. Re + 1/R0
1. 67%
2. 10%
3. 80%
4. 33%
5. 25%

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