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Unit 4 Immunology and
Public Health
Unit 4 – Immunology and Public
Health
1. The Immune
System
a)Non-specific
defences
b)Specific cellular
defences
2. Infectious
Diseases and
immunity
a)Transmission and
control
b)Active
Immunisation and
Vaccination and
the evasion of
immune responses
2. Infectious Diseases & immunity
b) Active immunisation, vaccination and evasion
By the end of this section you will be
able to …..
• state what is meant by herd immunity
• describe the benefit of herd immunity
to non-immune individuals
• describe the importance of herd
immunity in reducing the spread of
disease
• state that the herd immunity threshold
depends on a number of factors
What is Herd Immunity?
• http://www.immunisationscotland.org.uk
/why-immunise/benefits-for-you.aspx
What is Herd immunity?
• Herd immunity is where a large percentage
of a population are immunised.
• Therefore, they offer protection to nonimmune individuals as there is a lower
probability that they will come into contact
with infected individuals.
• This herd immunity is important in
reducing the spread of diseases and in
protecting vulnerable and non-vaccinated
individuals.
Herd immunity threshold
Disease
Transmission
Average number of
secondary infections
resulting from single
index case
Diptheria
Saliva
6–7
85%
Measles
Airborne
12 – 18
83 – 94 %
Mumps
Airborne
droplet
4–7
75 – 86 %
Whooping
cough
Airborne
droplet
12 – 17
92 – 94 %
Polio
Faecal-oral
route
5–7
80 – 86 %
Rubella
Airborne
droplet
5–7
83 – 85 %
Smallpox
Social
contact
6–7
83 – 85 %
Herd
immunity
threshold
Herd immunity threshold
• This is the percentage of the population
who need to be immunised by the
vaccine to offer protection for people
who are not vaccinated.
• Herd Immunity threshold varies as it
depends on:
– The disease
– The efficacy of the vaccine and
– The contact parameters for the population.
Can you now ….
• state what is meant by
herd immunity
• describe the benefit of
herd immunity to nonimmune individuals
• describe the importance
of herd immunity in
reducing the spread of
disease
• state that the herd
immunity threshold
depends on a number of
factors.
2. Infectious Diseases & immunity
b) Active immunisation, vaccination and evasion
By the end of this section you will be
able to …..
• give examples of barriers to herd
immunity in the developing world
• give examples of barriers to herd
immunity in the developed world
Is there a correlation?
There
is
of course
a correlation
(similar
Did the
pollen
count
cause
the cream
ice cream
sales toover
rise?
•• One
graphs
shows
ice
sales
• Did
the sale of ice cream cause the pollen count to
pattern)
year
and IS
theNO
other
showsTOpollen
• arise?
BUT
THERE
EVIDENCE
SUGGEST
• count.
Is
thereONE
any link?
THAT
CAUSES THE OTHER
The MMR Scare
• We know look at how this relates to the
MMR scare and why the MMR scare
became a barrier to herd immunity!
The MMR Scare
• Autism is a condition which affects
between 1-2 people in every 1000,
affecting neural development and
causing restricted and repetitive
behaviour.
• It affects social behaviour and
language; its causes are unknown.
• It is usually diagnosed from the age of
three onwards.
The MMR Scare
• A British doctor (Dr Wakefield) wrote a
report on 12 children who had been
vaccinated with the MMR vaccine and
were subsequently diagnosed as autistic.
• The result of this report was that
media interest was raised; many antiMMR stories appeared and there was a
significant fall in the number of children
given the MMR vaccine.
• What
happened to
the MMR
vaccination
rates of
children?
• What
happened to
the
incidence of
measles?
• Is there a
causal link?
NO Herd immunity to protect those who were not
vaccinated, so incidence of measles increases!
The MMR Scare
• It was subsequently established beyond
reasonable doubt that there is no causal link
between MMR vaccination and autism. The
doctor had a commercial interest in the
alleged link and was subsequently struck off.
• The scare affected no other countries; MMR
vaccination rates are rising again.
Watch the following video on the effect of adverse
publicity on the MMR vaccination
http://www.youtube.com/watch?v=jfheO9H8CD4
Importance of large scale studies
• The Madsen study in Denmark.
• Because Denmark tracks patients and the care they
receive they have been able to study the
correlation between vaccination and illness; the
data clearly shows that there is no correlation
between MMR vaccination and the incidence of
autism.
Why is this study more reliable than Dr Wakefield’s?
• The study was based on data from over half a
million children
• Over 440,000 had been vaccinated and there was
no greater incidence of autism amongst children
vaccinated as amongst those not vaccinated.
Barriers to herd immunity
• In most countries, policy in public health
medicine is to establish herd immunity
to a number of diseases.
• Difficulties can arise when widespread
vaccination is not possible due to
malnutrition and poverty (the developing
world) or when vaccines are rejected by
a percentage of the population (the
developed world).
House md clip - no vaccine
Can you now ….
• give examples of
barriers to herd
immunity in the
developing world
• give examples of
barriers to herd
immunity in the
developed world