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Perverse effects of vaccination
The perverse effects of vaccination require two conditions:
When too few are vaccinated the disease spreads more slowly than in an unvaccinated population. This raises the average age of infection, increasing the number of serious health problems associated with the disease.
Additional recommended knowledge
There is a critical threshold value (denoted qc) at which enough people are immune to the disease that its spread through the population (even to unvaccinated susceptible individuals) is stopped. This effect is commonly known as herd immunity.
If a vaccination programme does not attain qc, its effect is not to prevent the spread of the disease across the unvaccinated population; instead it delays the spread and so increases the average age at which individuals are infected. In some diseases that have an increased severity or risk of complications with increased age, therefore, such a vaccination programme may actually increase the number of deaths from and/or problems relating to the disease. Some epidemiologists also refer to this as an "epidemiological shift". These are the perverse effects.
Some infectious diseases that increase in severity with age:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Perverse_effects_of_vaccination". A list of authors is available in Wikipedia.|