The next important number is the reproduction rate (R). This is similar to the AR, but it looks at the number of people who get the disease from each infected person The higher the number, the more infectious the disease
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Mathematically, this gives us the minimum threshold for herd immunity as: R0*(1-HI) = 1 Where R = 1 and R0 is calculated based on the disease Turning this around, we get: HI =1-(1/RO)
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So to work out the threshold for herd immunity, we just have to solve this for each disease! Pretty cool, huh?
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What's the herd immunity threshold for mumps, based on this formula?
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Completing the formula, we get: HI = 1-(1/4) to 1-(1/7) = .75-.86 = 75-86%
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What this means is that we have to vaccinate 75-86% of the population to reach the threshold for herd immunity for mumps
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The more infectious the disease, the more you have to vaccinate Measles can live for hours, even days, outside the body, and has an R0 of >12, which means that we need to vaccinate 95% of the population to be sure it won't spread
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Smallpox only has an R0 of 7, which means that we only had to vaccinate ~85% of people to eliminate the disease
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And that's it. That's how herd immunity works Simple, easy, incredibly, wonderfully powerfulpic.twitter.com/GywhQILLPQ
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People do still get these diseases. No vaccine is 100% effective - although many are close to that - so vaccination rates often have to be higher than even the theoretical herd immunity threshold
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There are also a lot of people who have been scared away from vaccination by anti-vaccine activists who spread fear about lifesaving medical interventions
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But we know that vaccinations WORK Australia has now eliminated endemic measles and rubella, and almost entirely eliminated many other infectious diseases, with some of the highest vaccination rates in the world
#VaccinesWorkpic.twitter.com/MdFUNBp6U7
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So get your vaccines They will not only stop YOU from getting sick, they'll also stop you from passing diseases onto more vulnerable people like babies and immunocompromised people
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And the next time someone says "herd immunity is a myth!", feel free to reference this thread and/or blog Herd immunity is simple maths, nothing more, nothing less
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End of conversation
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