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GidMK's profile
Health Nerd
Health Nerd
Health Nerd
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@GidMK

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Health NerdVerified account

@GidMK

Epidemiologist. Writer (Guardian, Observer etc). "Well known research trouble-maker". PhDing at @UoW Host of @senscipod Email gidmk.healthnerd@gmail.com he/him

Sydney, New South Wales
theguardian.com/profile/gideon…
Joined November 2015

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    Health Nerd‏Verified account @GidMK 11 Dec 2018

    It's been 2 years since I posted this blog, and it's still one of my favourites. Here's a tweetorial on herd immunity, and why it's pretty cool #VaccinesWorkhttps://medium.com/@gidmk/herd-immunity-is-pretty-cool-adbc52630f9f …

    6:53 PM - 11 Dec 2018
    • 79 Retweets
    • 136 Likes
    • Yoshihiro Hayashi Nasir Ahmed isko1977 Lina Chervenkova Nikolay Dimitrov Saira Butt Jenna Coalson Mariela 🌸🍀眞葛原雪🍀🌸
    2 replies 79 retweets 136 likes
      1. New conversation
      2. Health Nerd‏Verified account @GidMK 11 Dec 2018

        First, let's get something out of the way: Herd immunity has nothing to do with cows! Saying "people aren't cows" is just displaying your ignorance for all to see

        1 reply 1 retweet 8 likes
        Show this thread
      3. Health Nerd‏Verified account @GidMK 11 Dec 2018

        So, what is herd immunity? At its most basic level, it is simply maths

        1 reply 1 retweet 4 likes
        Show this thread
      4. Health Nerd‏Verified account @GidMK 11 Dec 2018

        A more useful definition, however, is that herd immunity is the point at which enough people are immune to a disease to prevent non-immune people from getting itpic.twitter.com/px8N5xxZU1

        3 replies 4 retweets 14 likes
        Show this thread
      5. Health Nerd‏Verified account @GidMK 11 Dec 2018

        The first part of herd immunity is the attack rate (AR) of a disease. This is fairly simple - it's the rate of infection due to a disease in the population Mathematically, this means x/n = AR where x is the number of infected people and n is the total population

        1 reply 1 retweet 4 likes
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      6. Health Nerd‏Verified account @GidMK 11 Dec 2018

        If you have a school of 100 children, and 10 of them get measles, the attack rate would be

        1 reply 1 retweet 3 likes
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      7. Health Nerd‏Verified account @GidMK 11 Dec 2018

        The answer is fairly easy here - 10 children, total n of 100, so AR = x/n = 10/100 = 0.1 = 10% As I said, a simple metric of how infectious a disease is in practice

        1 reply 1 retweet 2 likes
        Show this thread
      8. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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

        1 reply 1 retweet 4 likes
        Show this thread
      9. Health Nerd‏Verified account @GidMK 11 Dec 2018

        (Sorry, small correction. The reproduction rate is usually abbreviated to R0, not R. R is the abbreviation for the EFFECTIVE reproduction rate, which we'll get to)

        1 reply 1 retweet 2 likes
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      10. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Here are some common R0s for various vaccine-preventable diseases, as per the CDC You can see why measles is such a problem!pic.twitter.com/ypkYMrciAp

        1 reply 3 retweets 4 likes
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      11. Health Nerd‏Verified account @GidMK 11 Dec 2018

        So this is the reproduction rate of the disease in an entirely vulnerable population. It's pretty simple - you look at each person, check their contacts - people they interacted with while infectious - and see how many get the disease What happens if people are immune?

        1 reply 2 retweets 3 likes
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      12. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Here's where the maths comes in. Now, immunity can be conferred by vaccines, but it doesn't HAVE to be. Even without vaccines you start seeing herd immunity once 90%+ of the population has been infected

        1 reply 2 retweets 3 likes
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      13. Health Nerd‏Verified account @GidMK 11 Dec 2018

        (The downside to this, of course, is that most people get sick and some die from the disease to earn that immunity, whereas vaccines are much safer)

        2 replies 2 retweets 4 likes
        Show this thread
      14. Health Nerd‏Verified account @GidMK 11 Dec 2018

        More The EFFECTIVE reproduction rate (R) is calculated based on the R0 Mathematically, it is: R0*y = R Where y is the proportion of the population susceptible to the disease

        1 reply 2 retweets 3 likes
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      15. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Based on the table above, what's the R for measles in a population with 75% vaccination rates?

        1 reply 1 retweet 3 likes
        Show this thread
      16. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Here's a more difficult question - based on the equation for R, what is the formula to work out the % for herd immunity (HI)?

        1 reply 1 retweet 3 likes
        Show this thread
      17. Health Nerd‏Verified account @GidMK 11 Dec 2018

        This is more complex because you have to think about what herd immunity really means Herd immunity means that each person passes on the disease to fewer than 1 person (on average), so the disease eventually peters out

        1 reply 2 retweets 6 likes
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      18. Health Nerd‏Verified account @GidMK 11 Dec 2018

        What this means, in technical terms, is that R<1 I.e. the effective reproduction rate in the population is less than 1

        1 reply 1 retweet 2 likes
        Show this thread
      19. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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)

        1 reply 2 retweets 4 likes
        Show this thread
      20. Health Nerd‏Verified account @GidMK 11 Dec 2018

        So to work out the threshold for herd immunity, we just have to solve this for each disease! Pretty cool, huh?

        1 reply 2 retweets 3 likes
        Show this thread
      21. Health Nerd‏Verified account @GidMK 11 Dec 2018

        What's the herd immunity threshold for mumps, based on this formula?

        1 reply 2 retweets 3 likes
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      22. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Completing the formula, we get: HI = 1-(1/4) to 1-(1/7) = .75-.86 = 75-86%

        2 replies 2 retweets 3 likes
        Show this thread
      23. Health Nerd‏Verified account @GidMK 11 Dec 2018

        What this means is that we have to vaccinate 75-86% of the population to reach the threshold for herd immunity for mumps

        1 reply 2 retweets 3 likes
        Show this thread
      24. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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

        1 reply 2 retweets 5 likes
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      25. Health Nerd‏Verified account @GidMK 11 Dec 2018

        Smallpox only has an R0 of 7, which means that we only had to vaccinate ~85% of people to eliminate the disease

        1 reply 2 retweets 5 likes
        Show this thread
      26. Health Nerd‏Verified account @GidMK 11 Dec 2018

        And that's it. That's how herd immunity works Simple, easy, incredibly, wonderfully powerfulpic.twitter.com/GywhQILLPQ

        1 reply 6 retweets 7 likes
        Show this thread
      27. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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

        1 reply 2 retweets 7 likes
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      28. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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 #VaccinesWork

        1 reply 2 retweets 5 likes
        Show this thread
      29. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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

        1 reply 3 retweets 6 likes
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      30. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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 #VaccinesWork

        1 reply 5 retweets 12 likes
        Show this thread
      31. Health Nerd‏Verified account @GidMK 11 Dec 2018

        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

        1 reply 6 retweets 9 likes
        Show this thread
      32. End of conversation

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