Perverse effects of vaccination

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.

Definition
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.

Examples

 * A rise in congenital rubella syndrome (CRS) cases in Greece following the introduction in 1975 of rubella vaccination for young children. This vaccination program failed because it did not attempt to protect adolescents and young women, and did not attempt to obtain high coverage, and shows that rubella vaccination programs should not be halfhearted.
 * A similar claim has been made for CRS the United States in the early 1970s, but this misrepresents the overall pattern of CRS incidence, which has fallen from an estimated 20,000 in the 1964 epidemic to 7 in 1983, with a large drop in CRS incidence the early 1980s and with rubella eliminated in the U.S. by 2004.

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