Transcript L15 Herd Immunity

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Immune system
diseases
Non specific
immunity
Disease survival
mechanisms
Physical &
chemical barriers
Infectious
Disease
Non-specific
Cellular response
Specific
immunity
Immunological
surveillance
T cells
B cells
Clonal Selection
theory
Transmission
Epidemiology
Vaccination
Public Health
We are here
Inflammatory
BIG PICTURE
LEARNING OUTCOMES
• EXAMINE THE IMPORTANCE OF VACCINE UPTAKE
• 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
• PROVIDE EVIDENCE FOR POTENTIAL PROBLEMS IN
PUBLIC HEALTH MEDICINE
HERD IMMUNITY
• ONLY THOSE WITH _____????______ HAVE THE BALL – NOW PASS IT
ONLY TO __________.
• EVERYONE ELSE TRY TO INTERVENE AND PREVENT THE BALL REACHING
THEIR TARGET
• HOW MANY TIMES DID THE BALL MAKE ITS TARGET?
Active Herd Immunity
Group protection from a disease.
If enough people are immunised,
the disease is less likely to be
transferred.
The activity modelled this (ball,
pathogen and the two people
trying to pass were nonvaccinated)
If one person is not immune they
are still in effect protected.
Eg. measles needs 92% of the
population vaccinated to achieve
active herd immunity.
HERD IMMUNITY
• COMPLETE THIS WORKSHEET TO ILLUSTRATE WHAT HAPPENS WHEN
IMMUNISATIONS IN A POPULATION CHANGE
• WHY WOULD THE
UPTAKE DROP?
HERD IMMUNITY
• MEMBERS OF A COMMUNITY WHO ARE NOT IMMUNE TO A
DISEASE ARE STILL PROTECTED, PROVIDED SUFFICIENT
NUMBERS OF PEOPLE IN THAT COMMUNITY ARE IMMUNE
• LITTLE OPPORTUNITY TO SPREAD AND FIND A NONIMMUNE PERSON
• ONLY APPLIES TO DISEASES THAT ARE CAUGHT FROM
OTHER PEOPLE!
DIPHTHERIA VACCINE
• BEFORE A VACCINATION PROGRAMME WAS INTRODUCED IN 1940, DIPHTHERIA
WAS A VERY COMMON CONDITION AND ONE OF THE LEADING CAUSES OF
DEATH IN CHILDREN.
• THE VACCINATION PROGRAMME HAS BEEN VERY SUCCESSFUL. SINCE 1986,
THERE HAVE BEEN ONLY 15 RECORDED CASES OF DIPHTHERIA IN ENGLAND AND
WALES, AND NO DEATHS. DIPHTHERIA IS A NOTIFIABLE DISEASE, WHICH MEANS
THAT IF A DOCTOR DIAGNOSES THE CONDITION, THEY MUST TELL THE LOCAL
AUTHORITY.
• EVEN THOUGH THE INCIDENCE OF DIPHTHERIA IN ENGLAND IS LOW, THERE'S A
RISK THAT AN OUTBREAK COULD OCCUR IF THE NUMBER OF PEOPLE WHO ARE
VACCINATED FALLS BELOW A CERTAIN LEVEL.
• THIS RISK WAS DEMONSTRATED BY THE DIPHTHERIA EPIDEMIC THAT STRUCK THE
COUNTRIES OF THE FORMER SOVIET UNION BETWEEN 1990 AND 1998. IT
RESULTED IN 157,000 CASES AND 5,000 DEATHS. THE EPIDEMIC WAS CAUSED BY
AN INCREASE IN THE NUMBER OF CHILDREN WHO WERE NOT VACCINATED
AGAINST THE DISEASE
INDIVIDUAL CHOICE ....
• MOST PEOPLE ARE WELL TO HAVE A VACCINE
• HOWEVER SOME PEOPLE IN THE COMMUNITY MAY BE NOT ELIGIBLE
FOR THE VACCINE (CANCER TREATMENTS OR IMMUNOSUPPRESSANT)
• THE MINOR NON-IMMUNE PEOPLE ARE PROTECTED BY THE MAJORITY
OF IMMUNISED PEOPLE
• THIS IS THE ONLY PREVENTION OF DISEASE WE CURRENTLY HAVE –
• WHEN IS A HERD NOT A HERD?
HERD IMMUNITY
THRESHOLD
• TO BE EFFECTIVE ONLY A SMALL MINORITY CAN BE UNVACCINATED
• PERCENT DEPEND ON DISEASE / VIRULENCE / TRANSMISSION MODE /
POPULATION DENSITY AND VACCINE EFFICACY
Disease
Transmission
Threshold for herd immunity
Diphtheria
Airborne saliva droplets
85%
Measles
Airborne
83-94%
Poliomyelitis
Faecal-oral route
80-86%
Whooping cough
Airborne
92-94%
Smallpox
Contact
80-85%
Rubella
Airborne droplets
80-85%
PUBLIC HEALTH MEDICINE
• IMMUNIZATION SO EFFECTIVE IT IS STILL THE MAIN PUBLIC HEALTH
POLICY FOR COMBATING DISEASE
• IN THE UK AND OTHER DEVELOPED COUNTRIES YOU HAVE SEEN THAT
VACCINATION UPTAKE DROPS DUE TO ADVERSE PUBLICITY
• IN DEVELOPING COUNTRIES WHERE HIGH INCIDENCE OF POVERTY AND
MALNUTRITION LARGE FACTORS MAY NOT BE A PRIORITY / COST
• WHO HELPS OUT – AIM TO ERADICATE POLIO – HOWEVER 2012
8.8MILLION CHILDREN DIED OF VACCINE-PREVENTABLE DISEASES!
DEMONSTRATION DEBATE
• HALF THE CLASS;
• ONE HALF WILL AGREE WITH THE STATEMENT (PRO)
• ONE HALF WILL DISAGREE WITH THE STATEMENT (OPPOSITION)
• IN YOUR HALVES COME UP WITH ARGUMENTS AND COUNTER
ARGUMENTS FOR YOUR SIDE – 8 MINUTES!
• NOMINATE 4 PEOPLE – 3 OF WHO WILL DELIVER THESE ARGUMENTS
AND THE FINAL PERSON WILL SUMMARISE
STRUCTURE (1 MINUTE EACH)
CLASS VOTE
• 1 ARGUMENT FROM PRO SIDE
• 1 COUNTER ARGUMENT FROM OPPOSITION
• 2ND ARGUMENT FROM PRO SIDE
• 2ND COUNTER ARGUMENT FROM OPPOSITION
• 3RD ARGUMENT FROM PRO
• 3RD COUNTER ARGUMENT FROM OPPOSITION
• FINAL SUMMARY FROM PRO
• FINAL SUMMARY FROM OPPOSITION
CLASS RE-VOTE
DEMONSTRATION DEBATE
• DR SOHAIL BHATTI, DIRECTOR OF PUBLIC HEALTH AT NHS EAST
LANCASHIRE, PROPOSES “TO BAR CHILDREN WHO HAVE NOT BEEN
VACCINATED FROM SCHOOL”
• IN AMERICA, WHERE THIS IS PUBLIC POLICY, THE INCIDENCE OF
CHILDHOOD DISEASE IS LOWER THAN IN THIS COUNTRY.
• USE EXAMPLES FROM P337-338
REVIEW
• WHAT IS HERD IMMUNITY?
• WHAT CAUSES THE DECREASE IN UPTAKE OF VACCINES?
REVIEW
What is herd immunity?
• Protect community
• Enough people immune less likely to spread
• Even if not had vaccine still protected
Causes of decrease in uptake?
• Malnutrition
•Poverty
•Adverse publicity / media scares / rejection of vaccine
SUMMARY SLIDE
HERD IMMUNITY
• IF A LARGE PERCENTAGE OF A POPULATION ARE IMMUNISED, NONIMMUNE INDIVIDUALS ARE PROTECTED 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.
• THE HERD IMMUNITY THRESHOLD DEPENDS ON THE DISEASE, THE
EFFICACY OF THE VACCINE AND THE CONTACT PARAMETERS FOR THE
POPULATION.
SUMMARY SLIDE
PUBLIC HEALTH MEDICINES
• 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
• WIDESPREAD VACCINATION IS NOT POSSIBLE DUE TO
POVERTY (THE DEVELOPING WORLD),
• WHEN VACCINES ARE REJECTED BY A PERCENTAGE OF THE
POPULATION (THE DEVELOPED WORLD EG. MMR).