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

March 5, 2020

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.


  • Reply Joe Gazaway March 4, 2020 at 2:30 pm

    I enjoyed the video! Keep it up! Would you like to be YouTube friends? :]

  • Reply Miracles Happen March 4, 2020 at 2:37 pm

    Great, its a year 1/2 away.

  • Reply Joe Gazaway March 4, 2020 at 3:00 pm

    Quite good video, awesome! Would you like to be YouTube friends? :]

  • Reply Joe Gazaway March 4, 2020 at 3:00 pm

    Brilliant work! Keep it up! Would you like to be YouTube friends? :]

  • Reply Ivan March 5, 2020 at 12:39 am

    Urm it's no longer an epidemic. It's a pandemic already

  • Reply Claudio Dinapoli March 5, 2020 at 11:44 am

    Good explanation. There's though another factor that should be counted when deciding how many people in a population should get a vaccine shot: the ratio of responder/vaccinated. A responder is someone who gets the shot and develops an effective immune response; on the contrary a non responder is someone who doesn't get immunized by the vaccine even though he/she gets a shot. The herd immunity must count the responders but not the non responders since only the formers slow down the spread of a microbe in a population. Every vaccine has a different percentage of responders. Some has a high percentage (such as measle vaccine) but a RO very high (18 assuming all people are susceptible). In other cases the percentage is not so high. As far as I remember BCG vaccine (Tuberculosis) has roughly 60% of responders.

  • Reply DrPardeep Kapoor March 5, 2020 at 6:11 pm

    No one has made it so clear. The explanation and examples are great. Medical Schools need teachers like you. Great job. Looking forward to more of such explanations

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