Before we talk about the importance of herd immunity we should define it. Herd immunity is sometimes called community immunity. I think herd immunity is most easily defined through an example.
Let's imagine two communities of 100 individuals.
Community A Most of the individuals have had vaccinations (90) and there are 10 people who are either too young or too immune compromised to be vaccinated. So in this community 90% of people have vaccinations and 10% cannot receive them.
Community B Few individuals have had vaccinations (10) and there are also 10 people in this community that are either too young or too immune compromised to receive vaccinations. In this community 10% have vaccinations, 10% are unable to get vaccinations and 80% are unvaccinated, meaning they have no immunity.
Now, let's imagine the virus that the vaccination is for comes into contact with these two communities.
In community A, 90% of people who come into contact with the virus are not effected by it because their immune system has antibodies. These 90% of individuals also do not become carriers, meaning once they come in contact with the virus they do not carry the virus off to infect other people. This means that 9 out of 10 times the virus tries to invade this community it is ineffective because of the immunity. Most of the herd/community is immune to the virus so those who don't have the immunity are unlikely to contract it.
In community B, 10% of the people who come in contact with the virus have immunity and do not become sick or carriers. However, 80%, or 8 out of 10, of individuals do come down with the sickness and carry the virus into the community to infect others. That means the 10% unable to be vaccinated have an 80% chance of catching this virus. Those most susceptible to the virus are highly likely to come in contact with a community member who is carrying the virus.
This is the basis of herd immunity, the more of the herd that is immune the more safeguard there is for those who are unable to receive vaccinations or may be in serious danger from becoming sick, such as the young, those with immunodeficiency diseases, or undergoing chemotherapy. Herd immunity works on small scales; a single family with a new baby or a sick elder; or a community as large as an entire city, state, country or the broader world. Herd immunity is important to stop the spread of a disease from one individual to another and is a major force in preventing epidemics and pandemics.
What percentage (R0) of the population must be vaccinated to provide herd immunity?
This is a very tricky question and involves several variables. One major variable is how contagious the virus or bacteria, or how fast it spreads. The more contagious the disease the higher the percentage of people needed to provide herd immunity. The interaction and travel patterns of the community matters to this as well. With modern travel, such as cars and airplanes, someone carrying a virus can travel much farther in a single day. This means that the herd immunity of the home population and the destination population are both equally important when it comes to a carrier. In general, the higher the percentage who are vaccinated, the better the herd immunity.(Check out the link in the more information section for an article about how this is calculated)Why are vaccinations important to herd immunity?
Vaccinations provide a few important things. First, immunizations create a record of who has immunity. This information is important during an outbreak and also allows officials to track down patient Zero. (Patient Zero is a term for the person who initially brings a disease into a community. Knowing patient zero can allow officials to track who they have been in contact and who might have contracted the illness.) Secondly, vaccinations allow the individual and the medical professionals to know that an individual, or group of individuals, is immune to a specific strain of a disease. Perhaps the most important aspect of herd immunity through vaccinations is that it allows a large portion of the population to become immune without having to contract the disease. This is a particularly important benefit when the disease is debilitating or has a high rate of death, such as polio. A final importance, at least to those who have and love pets, is that some diseases are zoonotic, meaning they can be transmitted between people and animals. Having a pet and/or person vaccinated against a zoonotic disease protects the human population, the pet population, and in some cases populations of food animals.A great example of a pet vaccination that provides safety to both the pet and the humans who love them is the rabies vaccination.More information:
Research paper on the benefits of herd immunity in Meningitis
PBS page that discusses how to calculate percentages required for herd immunity to be effective as well as other related topics.
CDC discussion of herd immunity
Series of interactive animations to show visually the impact of herd immunity vs no herd immunity
Free Courses online that cover immunity and health
An Introduction to Global Health
An Introduction to Population Health
Epidemiology: The Basic Science of Public Health
Epidemics, Pandemics, and Outbreaks
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