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  • When a new disease emerges and starts

  • infecting people in a population,

  • one of the things we really want to know

  • is whether it's going to continue to spread

  • and infect more and more people,

  • or whether it's eventually going to die out

  • Some diseases like measles are highly infectious

  • In a fully susceptible population, each measles case

  • will on average infect about 16 to 18 additional people

  • Something like flu, however, is less transmissible

  • each case, on average, would infect between 2 to 3 people

  • The number of cases that each infectious person generates

  • can vary for different diseases

  • and we call this number the Basic Reproduction Number,

  • or R0 for short

  • R0 doesn't depend on how severe the symptoms are,

  • rather it's a measure of how transmissible the infection is

  • and as a result we can use it to work out

  • what's required to stop an epidemic

  • One of the ways we can stop epidemics is using vaccination

  • Now you might think that to stop a disease

  • you need to vaccinate the entire population

  • but actually this isn't the case

  • Of course if you vaccinate someone

  • it protects them, and stops them from getting infected

  • But because people who are vaccinated can't pass it on,

  • vaccination also stops the chains of transmission

  • and that means that this can create a protective barrier

  • which actually stops the epidemic

  • spreading within a population

  • Say the basic reproduction number of an infection is 2

  • This means that in a fully susceptible population,

  • each infected person will on average

  • give the disease to 2 other people

  • But if 50% of the population are vaccinated,

  • each infectious person on average

  • will only be able to give it to 1 of these 2 people

  • and this means that the epidemic

  • wouldn't be expected to grow over time

  • This is known as herd immunity

  • Herd immunity means that long as a certain proportion

  • of the population is vaccinated,

  • the disease won't be able to transmit within that population

  • We've seen that if the basic reproduction number, R0, is 2,

  • we need to vaccinate half the population

  • to stop disease transmission

  • By the same logic, if R0 is 3,

  • we need to vaccinate two-thirds of the population

  • If we keep going, for highly infectious diseases like measles,

  • we need to vaccinate 17 out of every 18 people

  • or 94% of the population to stop transmission

  • Herd immunity is especially useful for

  • protecting members of the population

  • who can't be vaccinated

  • perhaps because they're too old, too young, or have weak immune systems

  • If they're surrounded by people who have received the vaccine

  • then that can protect them from infection

  • If however people forego vaccination

  • then the herd can no longer protect these people

  • This means that the population as a whole

  • can be vulnerable to outbreaks

  • So far, we've been talking about averages,

  • one of the big challenges in my work

  • is incorporating some of the complexities of reality

  • into these mathematical models

  • Vaccines are an incredibly powerful public health tool

  • and the best way to protect populations against disease

  • is to make sure as many people are vaccinated as possible

  • so we don't run the risk of slipping below

  • this vaccine threshold for herd immunity

When a new disease emerges and starts

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B2 US population herd vaccinate herd immunity vaccinated immunity

What is Herd Immunity?

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    Sophie posted on 2020/04/27
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