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  • Welcome to this Medmastery coronavirus update, I'm Franz Wiesbauer.

  • I'm an internist, trained in epidemiology and public health at Johns Hopkins, and

  • the founder of Medmastery, where we teach important clinical skills to doctors and

  • other healthcare providers around the world.

  • Today we're going to talk about herd immunity and how to stop an epidemic.

  • Let's get started.

  • So what is herd immunity?

  • Well, it's a basic principle that's used in combating epidemics.

  • Herd immunity occurs when a significant proportion of the population or the herd

  • have been vaccinated or are immune by some other mechanism, resulting in protection

  • for susceptible individuals.

  • Let's see how this works in action.

  • In a previous video we talked about R naught, the basic reproductive ratio.

  • Let's look at this example where we have one diseased or infected individual and 18

  • susceptible individuals.

  • As you might remember, R naught is the average number of individuals an infected

  • person gives the disease to.

  • R naught is fairly constant for a given disease.

  • R naught for the novel coronavirus SARS-Co-2 has been estimated to range

  • somewhere between two and three.

  • So let's say this COVID-19 patient gives the disease to these three individuals.

  • And let's say these three individuals give the disease to three other individuals in

  • turn.

  • Then this is what the situation will look like after some time.

  • Now, let's assume that we vaccinated some of these susceptible populations, such

  • that they became immune to the virus.

  • Now the virus can't infect individuals it infected in the previous scenario.

  • Now our index case only infects one other person, and due to the immunity or herd

  • immunity of the group, this newly infected case can also only infect one other

  • person.

  • What we've done here is to reduce the basic reproductive ratio of three to an

  • effective reproductive ratio of one.

  • As we've seen in a previous video, when R is equal to one, the disease remains

  • stable and won't grow.

  • The herd immunity threshold is the proportion of a population that needs to

  • be immune in order for an infectious disease to become stable in that

  • community.

  • Or in other words, in order for R to become equal to or lower than one.

  • If this is reached, for example, through immunization, then each case leads to a

  • single new case and the infection will become stable within that population.

  • So how do we know what proportion of the population needs to be immunized in order

  • to reach herd immunity?

  • Let's look at a disease with an R naught of eight, or one infected individual

  • infects eight others on average.

  • What would need to happen for them to only be able to infect one other person?

  • Well, we would need to immunize these seven over here.

  • So seven divided by eight or seven eighths is the herd immunity.

  • It's calculated as R naught minus one divided by R naught.

  • As we've learned previously R naught for SARS-KoV-2 is between two and three.

  • So how many people would we have to vaccinate, if there was a vaccine in order

  • for the epidemic to stop?

  • Well, that's two minus one divided by two, which is one half or 50% if R naught was

  • assumed to be equal to two.

  • And three minus one divided by three or two thirds if R naught was assumed to be

  • equal to three.

  • So according to this calculation, we'd have to vaccinate between 50 and 66% of

  • the population.

  • Now, I recently heard Marc Lipsitch an epidemiologist from Harvard mention in a

  • podcast interview that the herd immunity of COVID-19 according to his data, was

  • around 40%.

  • I'm sure he has more complex tools to factor in other variables that could

  • influence herd immunity.

  • But if you want to go by the books, the calculation of herd immunity according to

  • the formula we provided is valuable and valid.

  • By the way, I really recommend you follow Marc on Twitter.

  • He provides great insights about the epidemic and seems to be a super smart

  • guy.

  • That's it for now.

  • If you want to improve your understanding of key concepts in medicine and improve

  • your clinical skills, make sure to register for a free Medmastery trial

  • account, which will give you access to free videos, downloads, and updates.

  • We'll help you make the right decisions for yourself and your patients.

  • If you like this video, make sure to subscribe to our YouTube channel so you'll

  • get notified when we publish new videos.

  • See you soon.

Welcome to this Medmastery coronavirus update, I'm Franz Wiesbauer.

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B2 US herd herd immunity naught immunity disease infected

COVID-19 Update 2: How to stop an epidemic - Herd immunity

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    Sophie posted on 2020/04/27
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