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The perverse effects of vaccination require two conditions:

1. Too few susceptibles are vaccinated against 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.

Contents 1 Definition 1.1 Examples 1.2 Diseases 2 See also 3 References

There is a critical threshold value (denoted q_c) 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 q_c, 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.^[1]

• The rise in congenital rubella syndrome cases in the United States ^[2] and Greece ^[3] following the introduction of rubella vaccination for young children have been cited as examples of the perverse effects of vaccination.
• In the United States, a mumps epidemic that began in December 2005 was found to be caused by an inaccurate estimation or reporting of vaccine efficacy, resulting in the highest number of cases in post-pubescent individuals.

Some infectious diseases that increase in severity with age:

• Orchitis is a complication of mumps more common in post-pubescent males
• Polio is more likely to be paralytic in older people
• Rubella can cause complications in pregnancy
• Chickenpox has a number of complications in older patients, including pneumonia

• Mathematical modelling in epidemiology for the mathematics of perverse effects
• Unintended consequences

1. ^ Roy M. Anderson, Robert M. May, B. Anderson (1992). Infectious Diseases of Humans: Dynamics and Control. Oxford Univ. Press, Oxford. ISBN 0-19-854040-X. 
2. ^ Catherine J. M. Diodati (1999). Immunization: History, Ethics, Law and Health. Integral Aspects Inc. ISBN 0-9685080-0-6. 
3. ^ Panagiotopoulos, T; Antoniadou I, Valassi-Adam E (1999). "Increase in congenital rubella occurrence after immunisation in Greece: retrospective survey and systematic review". BMJ 319: 1462-1466.