What is Herd Immunity?

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


  1. Although the evidence for vaccination-based herd immunity is yet to materialize, there is plenty of evidence to the contrary. Just a single publication by Poland & Jacobson (1994) reports on 18 different measles outbreaks throughout North America, occurring in school populations with very-high vaccination coverage for measles (71% to 99.8%). In these outbreaks, vaccinated children constituted 30% to 100% of measles cases.

  2. Still don’t understand why people who were vaccinated TWICE for the mumps/measles are contracting those diseases 🤔

  3. Herd immunity is a big lie. Based on something from 1934- it’s just a theory and it doesn’t prevent measles outbreak. Measles only killed 1 in 10,000 but we have NO idea about the negative ramifications of giving our kids some drug from big pharmaceutical who just want money money money

  4. "According to the CDC, it appears that adult vaccination rates for most vaccines ARE BELOW 50%." https://childrenshealthdefense.org/news/herd-immunity-a-dishonest-marketing-gimmick/

    So much for 'herd immunity' in the US. Where's all the outbreaks then? (and where have they been all these years without the required minimum percentage of vaccinated individuals in order to accomplish 'herd immunity'? – say, between 90 and 100%, according to 'science' 😉

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