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. In epidemiology a susceptible individual (sometimes known simply as a susceptible) is a member of a population who is at risk of becoming infected by a disease, if they are exposed to the infectious agent. ... Vaccination is a term coined by Edward Jenner for the process of administering a weakened form of a disease to patients as a means of giving them immunity to a more serious form of the disease. ...

Definition

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. The immune system is the system of specialized cells and organs that protect an organism from outside biological influences. ... The effectiveness of a vaccine depends, amongst other things, on the percentage of the population which has received it and is still within the period of protection offered by that vaccine. ...

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 problems relating to the disease. Some epidemiologists also refer to this as an "epidemiological shift". These are the perverse effects.

Examples

• Diodati(1999) suggests that the rise in Congenital rubella syndrome cases in the United States following the introduction of Rubella vaccination for young children is an example 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.

Congenital rubella syndrome (CRS) can occur in a developing fetus of a pregnant woman who has contracted rubella during her first trimester. ... Rubella (also known as epidemic roseola, German measles, liberty measles or three-day measles) is a disease caused by the Rubella virus. ...

Diseases

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

Orchitis is an often very painful condition of the testicles involving inflammation, swelling and frequently infection. ... Poliomyelitis (polio), or infantile paralysis, is a viral paralytic disease. ... Rubella (also known as epidemic roseola, German measles, liberty measles or three-day measles) is a disease caused by the Rubella virus. ... Congenital rubella syndrome (CRS) can occur in a developing fetus of a pregnant woman who has contracted rubella during her first trimester. ... Chickenpox, also spelled chicken pox, is the common name for Varicella simplex, classically one of the childhood infectious diseases caught and survived by most children. ... Pneumonia is an illness of the lungs and respiratory system in which the microscopic, alveoli (air-filled sacs) responsible for absorbing oxygen from the atmosphere become inflamed and flooded with fluid. ...

See also

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

It is possible to model mathematically the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination. ... Unintended consequences can be either positive, in which case we get serendipity or windfalls source of problems, according to the Murphys law definitively negative: perverse effect, which is the opposite result to the one intended The Law of Unintended Consequences holds that almost all human actions have at least...

References

• Catherine J. M. Diodati (1999). Immunization: History, Ethics, Law and Health. Integral Aspects Inc. ISBN 0968508006.
• Roy M. Anderson, Robert M. May, B. Anderson (1992). Infectious Diseases of Humans: Dynamics and Control. Oxford Univ. Press, Oxford. ISBN 019854040X.