Perverse effects of vaccination

The perverse effects of vaccination require two conditions:

1. Too few susceptiblesare vaccinatedagainst an infectious disease.
2. The severity of the disease increases with age.

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.

Definition

There is a critical threshold value (denoted q_c) at which enough people are immuneto 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 q_c, its effect is not to prevent the spread of the disease across the unvaccinated poupulation; 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 problems relating to the disease. These are the perverse effects.

Examples

• Diodati(1999) suggests that the rise in Congenital rubella syndromecases in the United States following the introduction of Rubellavaccination for young children is an example of the perverse effects of vaccination.

Diseases

Some infectious diseases that increase in severity with age:

• Orchitisis a complication of mumpsmore common in post-pubescent males
• Poliois more likely to be paralytic in older people
• Rubellacan cause complications in pregnancy
• Chickenpoxhas a number of complications in older patients, including pneumonia

See also

• Mathematical modelling in epidemiologyfor the mathematics of perverse effects
• Unintended consequences

References

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This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Perverse+effects+of+vaccination Wikipedia article Perverse effects of vaccination.