The term herd immunity is often heard in the news in regard to vaccinations.
At an individual level, vaccines prevent people from getting a specific disease, be it measles or polio.
But at the population level, it is not always necessary for everyone
in the population to have immunity to a disease to prevent a widespread
epidemic in the population. This is because there is a certain
threshold of immunity in a population that prevents a disease from becoming widespread.
Let’s take a closer look at how this happens.
exposed from the environment. In any given population
there are 3 groups of people with regard to any
specific disease-causing organism. There are those who are
currently infected with the organism, those who are capable of contracting the
disease and those who are immune to it.
In cases of communicable diseases an epidemic begins when one
person is infected and becomes contagious or is capable of spreading the
disease to others. For example, each year we experience an
epidemic of chickenpox and influenza in the general population.
The infected individual comes into contact with an individual who does not
have the disease but is not immune to it either.
An exposure event can be breathing the air in a room where someone is sneezing or coughing
or it can be direct contact with the infected person.
Either way an exposure occurs and the susceptible
person now has the organism in their system.
At this stage the exposed individual may fight off the disease and become
immune without showing symptoms. Exposure does not automatically
mean infection with symptoms. However, most infectious diseases are
categorized as diseases because they’re quite good at making us sick
and causing illness.
We’ll assume this is the case here with the chickenpox exposure.
At this stage the second person becomes sick and infectious.
This person can go on to expose another healthy but susceptible person
to the infectious organism. That person then becomes sick and infectious.
Then another susceptible individual becomes exposed and then becomes sick themselves.
and the process continues on and on. In this way the spread of
the disease-causing organism quickly moves through the population
creating a chain of infection that continues unabated.
So, if the point of medical intervention is to prevent such
chains of infection, how is this accomplished? Lets’ start again with
an initial sick individual.
This person is able to pass the disease-causing organism on to a
new susceptible person. The second individual is now sick
and capable of passing the disease on to another. However, in this case,
the next individual in the chain
is already immune to this particular infectious organism.
In this example, the person had either already had chicken pox, gotten better
and is now immune or they were vaccinated and have become immune.
At this point the chain of infection is broken and no more people are exposed or become sick. In other words, no epidemic occurs.
are exposed or become sick. In other words, no epidemic occurs.
Now let’s consider a more realistic scenario and see how diseases
are transmitted through a population.
Here we have one sick individual who comes into contact with 4
other people who are not immune to this infectious organism.
After all, this is a more likely scenario, someone who is sick can come into
contact with many people. We’ll only consider 4 here
though in reality it could be dozens of people, say in a classroom.
One sick child can expose 20 other students in elementary
school or perhaps a couple hundred in a college lecture hall.
Of the 4 people exposed, each of them becomes infected,
shows symptoms and can now expose another 4 people to the disease.
This next group of people becomes sick and goes on to infect
another 4 individuals who then go on to infect another 4 people.
In the end, since everyone in the population is susceptible,
everyone comes down with the disease. This may not be a problem
for chickenpox where the incidence of death is extremely low
-but say this is the pattern for Ebola, where more than 80 or 90%
will die without extensive medical intervention.
High mortality rates present a serious problem
when everyone exposed can become sick.
So what happens when we start to account for a small number of people in the
population who happen to already be immune to the infectious organism?
In this case there are only a few immune individuals in the population.
Will this be enough to protect the population as a whole?
The initial patient exposes 4 people to the disease, however,
one of these 4 is immune.
That means that all of the people down the line who otherwise would have been
exposed to the infectious organism, now will not be exposed.
However, there are 3 infected individuals who can now spread the
disease onward. Of the 3 sick individuals, only two are able to pass
on the infectious organism because one of the next 3 in the chain
happens to be immune. The disease is blocked from spreading. Now there are only 2 sick individuals to pass the disease on to the next individuals in the chain. However, of the two sick individuals, only one happens to pass along the disease because the other has exposed an immune individual who cannot become infectious and spread the disease further. Now there is only one person left who is infectious and who manages to spread the illness to one last person.
Now there are only 2 sick individuals to pass the disease on to the next
individuals in the chain. However, of the two sick individuals,
only one happens to pass along the disease
because the other has exposed an immune individual who cannot become
infectious and spread the disease further.
Now there is only one person left who is infectious and who
manages to spread the illness to one last person.
Now the outbreak has run its course but look at the results. Six individuals
were prevented from even coming into contact with the illness thanks to the few who were immune.
They broke the chain of infection. However, seven individuals
did come down with the illness,
more than were protected. This would be considered an epidemic outbreak.
If this illness were a fatal one, almost half of the population would have died.
Let’s see what happens in a case where we have a mere 50% immunity
in the population - a level considered far below effective levels of
herd immunity. We begin with our first infected individual
who exposes 4 other people. In this case, half of the individuals
exposed to the infectious agent are already immune.
However the other half do become sick and infectious.
Of these two only one is able to pass the infection on to a susceptible
individual as the other exposed individual is immune.
At this stage the remaining infected patient has a 50/50 chance of
exposing the infectious agent to a susceptible individual.
In this example the next person happens to be immune
so the chain of infection is broken and the epidemic is thwarted.
Even at 50% herd immunity, only 3 members of the population become sick.
There are many more individuals who were capable of becoming infected
but since they are surrounded by others who are immune, they never became exposed,
thus they never became ill. Still, having about 20% of your population
dying off would be considered unacceptable.
Ideally, it is most effective to have herd immunity in the 85 to 95% range.
When this level of immunity is achieved the odds are pretty low of an infected
individual being able to expose a susceptible person to the disease. Even if this does happen, the odds are again low of that second person being able to find and infect a third one. Even in situations where a few cases appear the disease cannot spread to the population as a whole and cause a massive outbreak when herd immunity is sufficiently high.
Even if this does happen, the odds are again low of that second person
being able to find and infect a third one. Even in situations where a
few cases appear the disease cannot spread to the population as a whole and
cause a massive outbreak when herd immunity is sufficiently high.