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.
What is the herd immunity threshold (HIT) for measles?
How do you calculate the herd immunity threshold?
What is the herd immunity threshold for a disease with R0 = 3?
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