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18.06.2013
1
Vaccines:A success story with failures
Ursula WiedermannInstitute of Specific Prophylaxis and Tropical Medicine, Medical
University Viennawww.meduniwien.ac.at/tropenmedizin
Aims of vaccination
Individual protection!(protection againstinfection/disease)
Collective protectionHerd immunity;
reduction of transmission
Reduction of incidence of disease
– prevention of epidemics
Elimination(reduction of infection in certain geographic area;
protection of re-emergeringdiseases)
Eradication(premanent worldwide
elimination of pathogen)
High vaccinationcoverage
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Benefit-Risk, Benefit-Cost Ratio
• Benefit-Risk Ratio (important for individual consideration):– Evaluation of the risk of disease and the risk of vaccination with
respect to the same risk parameter (death, sequelae) within a certain time interval:
– For all existing vaccines the calculation of the benefit/risk ratio is positive
• Benefit-Cost Estimation (for implementation of vaccine programs) :– is calculated to document social-economic savings through high
vaccination coverage: the benefit-cost ratio is positive for diseases with high incidence and high morbidity;
– For diseases with high individual risk for morbidity/mortality but low incidence this calculation is not always positive
Ehret J, Vaccine 2003
Public health issues of vaccination programs
Collective protectionHerd immunity;
reduction of transmission
Reduction of incidence of disease
– prevention of epidemics
Positive benefit-costratio:
• Reduction of therapycosts
• Reduction of hospitalisation rates• Reduction of
prevalence andincidence
• Number of gained lifeyears
• Number of gainedworking days
High vaccinationcoverage
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Definition of efficacy versus effectiveness
„Efficacy“:Measures the protective effect of a vaccine withrespect to a defined endpoint and selected studypopulation = ideal study situation (non „real lifessituation“)
„Effectiveness“Measures the protective effect of a vaccine underreal life conditions – non selected population, non controlled (for age, disease, preexisting immunityetc - direct, indirect, total, overal effects)
Development of human vaccines
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Impact of vaccination on epidemiology of infections
Cost benefit is the sum of direct and indirect effects
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Indirect effects - Herd immunity
= Protective value of a vaccine is due to:– Direct protective effect of a vaccine– Number of vaccinated people
– Protection of unvaccinated– Improved protection of vaccinated– Improvemet of cost-benefit ratio
Leads to:
Herd immunity/protection
Results from:
Reduction of pathogen transmission in a populationthat is protected to a certain percentage via vaccination („herd protection“):
• Only possible with human to human transmission;• Possible with live or inactivated vaccines
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Erreger-Transmission von Mensch zu Mensch
„Transmissability“ = Übertragungskapazität eines Erregers wird durch die „B asisreproduktionszahl“ (R0) definiert:
= Anzahl an Personen in einer ungeschützten Population, die durch 1 Infizierten angesteckt werden
Hängt ab von:
– Biologischen Eigenschaften des Erregers (Pathogenität, Virulenz)
– Häufigkeit des Erregerkontakts und Art des Kontakts
– Umweltbedingungen (Temperatur, Kontaktbedingungen wie Schule, Altersheim, Haushalt, Immunstatus, etc)
„Herd immunity threshold“= Mindestdurchimpfungsrate die zur Verhinderung einer Epidemie ausreicht (unter der Annahme, dass Vakzine 100% wirksam)
Epidemic
Vaccinationrate: 75%
Prevention ofepidemic
Rn= Netto Reproductionsnumber; 1
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Infektion Basisreproduktionszahl (R0)
Durchimpfungsrate (%)*
Diphtherie 6-7 85
Influenza 1,2-2 40-50
Masern 12-18 92-94
Mumps 4-7 75-86
Pertussis 12-17 92-94
Polio 2-15 50-93
Röteln 6-7 83-85
Varizellen 8-10 88-90
*Assuming that the vaccine efficacy is 100% – if it < 100%, vaccination coverage needs to be higher
Correlates of vaccine-induced immunity