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Herd immunity occurs when most of a population is immune to an infectious disease and thus
provides indirect protection to those that are not immune to the disease. For example, if 90% of
a population is immune to a virus, nine out of every ten people who encounter someone with the
disease won’t get sick (and won’t spread the disease any further). In this way, the spread of
infectious diseases is kept under control. Depending how contagious an infection is, usually 70%
to 90% of a population needs immunity to achieve herd immunity.
However, there are some “caveats” to this concept (presented below).
Homogenous mixing: In a given population, individuals must have the same opportunity
(chance) to interact with another individual for herd immunity to protect everyone. This doesn’t
always work. Think about who you “hang out” with. You tend to associate with others that are
more like you (e.g., age, interest, occupation, culture, ethnicity). You likely don’t interact with
others (as much) that are very different from you. So, if you choose not to vaccinate, chances are
you associate with others that likely have the same or similar ideology or behaviors. Thus, we
often see outbreaks of vaccine preventable disease in communities that have low vaccination
rates. See recent example in Asheville, NC regarding
chickenpox: https://www.nytimes.com/2018/11/20/health/chicken-pox-vaccineasheville.html (Links to an external site.)
Immigration/Emigration: Outwards migration (emigration: leaving one’s country) and
inwards migration (immigration: coming into a foreign country) rates are also import for herd
immunity concepts. If you have movement of too many non-immunized people into a
population, you decrease your herd immunity “protection”. This doesn’t just apply at a country
level, it applies at a community level more commonly.
Population structure: Recall our conversations about the population structures. Herd
immunity works better when populations have an age profile like a “high income” country
(vertical sides) and works less well when the population is more pyramid shaped (“low income”
countries). This is because there are so many more young people at the base of the pyramid in
low income countries and many (sometimes most) of these children do not have complete
immunity. As populations get older, they tend to have more people with immunity (natural or
vaccination).
R0 (Secondary Spread Rate): Perhaps the most important concept is the R0 (called “Rnaught”). This is the rate of secondary spread which is defined as the number of susceptible
(non-immune) people one infected case will infect. If the R0 is 5, then there will be 5 secondary
(additional) infections for each case. The level of immunity needed to reach herd immunity if
generally calculated as: Himmunity = 1- (1/ R0). Thus, if a disease has an R0 of 5, the herd
immunity level must be at least 0.8 (or 80%). See the below table for some real-life examples.
Disease
Transmission Route R0
Calculated Herd
immunity threshold
Diphtheria (Links
to an external
Saliva
site.)
Measles (Links
to an external
Airborne
site.)
Mumps (Links to
Airborne droplet
an external site.)
Pertussis (Links
to an external
Airborne droplet
site.)
Polio (Links to
Fecal-oral route
an external site.)
Rubella (Links to
Airborne droplet
an external site.)
6-7
85%
12-18
83 – 94%
4-7
75 – 86%
12-17
92 – 94%
5-7
80 – 86%
5-7
80 – 85%
See this article for additional information: https://abcnews.go.com/Health/r0-covid-19-viruskey-metric-opening-plans/story?id=70868997 (Links to an external site.)
Using the below document, complete the questions (last page) and submit through Canvas
Herd Immunity Analysis-1.docx
Download Herd Immunity Analysis-1.docx
Herd immunity occurs when most of a population is immune to an infectious disease and thus
provides indirect protection to those that are not immune to the disease. For example, if 90%
of a population is immune to a virus, nine out of every ten people who encounter someone
with the disease won’t get sick (and won’t spread the disease any further). In this way, the
spread of infectious diseases is kept under control. Depending how contagious an infection is,
usually 70% to 90% of a population needs immunity to achieve herd immunity.
However, there are some “caveats” to this concept (presented below).
Homogenous mixing: In a given population, individuals must have the same opportunity
(chance) to interact with another individual for herd immunity to protect everyone. This
doesn’t always work. Think about who you “hang out” with. You tend to associate with others
that are more like you (e.g., age, interest, occupation, culture, ethnicity). You likely don’t
interact with others (as much) that are very different from you. So, if you choose not to
vaccinate, chances are you associate with others that likely have the same or similar ideology or
behaviors. Thus, we often see outbreaks of vaccine preventable disease in communities that
have low vaccination rates. See recent example in Asheville, NC regarding chickenpox:
https://www.nytimes.com/2018/11/20/health/chicken-pox-vaccine-asheville.html
Immigration/Emigration: Outwards migration (emigration: leaving one’s country) and inwards
migration (immigration: coming into a foreign country) rates are also import for herd immunity
concepts. If you have movement of too many non-immunized people into a population, you
decrease your herd immunity “protection”. This doesn’t just apply at a country level, it applies
at a community level more commonly.
Population structure: Recall our conversations about the population structures. Herd
immunity works better when populations have an age profile like a “high income” country
(vertical sides) and works less well when the population is more pyramid shaped (“low income”
countries). This is because there are so many more young people at the base of the pyramid in
low income countries and many (sometimes most) of these children do not have complete
immunity. As populations get older, they tend to have more people with immunity (natural or
vaccination).
R0 (Secondary Spread Rate): Perhaps the most important concept is the R0 (called “R-naught”).
This is the rate of secondary spread which is defined as the number of susceptible (nonimmune) people one infected case will infect. If the R0 is 5, then there will be 5 secondary
(additional) infections for each case. The level of immunity needed to reach herd immunity if
generally calculated as: Himmunity = 1- (1/ R0). Thus, if a disease has an R0 of 5, the herd immunity
level must be at least 0.8 (or 80%). See the below table for some real-life examples.
Transmission Route
R0
Calculated Herd
immunity threshold
Diphtheria
Saliva
6-7
85%
Measles
Airborne
12-18
83 – 94%
Mumps
Airborne droplet
4-7
75 – 86%
Pertussis
Airborne droplet
12-17
92 – 94%
Polio
Fecal-oral route
5-7
80 – 86%
Rubella
Airborne droplet
5-7
80 – 85%
Disease
See this article for additional information: https://abcnews.go.com/Health/r0-covid-19-viruskey-metric-opening-plans/story?id=70868997
Data Analysis:
Question 1: If a novel pathogen has an R0 of 2.5 (similar to COVID-19 coronavirus estimates),
what is the level of herd immunity needed (assuming the above caveats) to protect the
population? Please provide as a percentage.
Answer:
Question 2: What real-world limitations will there be in reaching your estimate in the United
States?
Answer:
Question 3: In addition to using a vaccine, what other method of establishing immunity will be
“at play” to reach herd immunity levels?
Answer:

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