Perverse effects of vaccination

Perverse effects of vaccination occur when a vaccination program causes more harm than it cures. This can happen if too few are vaccinated, allowing the disease to spread, although more slowly than in an unvaccinated population. This raises the average age of infection, which in some cases can increase the number of serious health problems associated with the disease.
Theoretical models
In mathematical modelling in epidemiology, there is a critical threshold value (denoted q<sub>c</sub>) 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 program does not attain q<sub>c</sub>, 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. This is called an epidemiological shift. As an extreme example, suppose a disease spreads so rapidly that everybody is typically exposed to it once a day; people of all ages will contract the disease. In contrast, if everybody is typically exposed to the disease only once every 30 years, most children will not have the disease. In diseases like rubella and mumps, which have an increased severity or risk of complications with increased age, a vaccination program that causes an epidemiological shift can in some cases have the unintended consequence of increasing the number of deaths and problems caused by the disease, even if it protects vaccinated individuals.
Theoretical models for vaccination programs have found that perverse effects of vaccines are possible in other circumstances. One example can occur if a vaccine targets one tuberculosis (TB) strain that provides cross-immunity against a non-targeted TB strain that in turn does not provide much immunity.
Examples
Perverse effects arose 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. The resulting epidemiological shift caused rubella to infect more pregnant women and cause more CRS, showing that rubella vaccination programs should not be halfhearted. A claim has been made that similar perverse effects occurred in the U.S. in the early 1970s, but this misrepresents the overall pattern of U.S. CRS incidence, which fell from an estimated 20,000 in the 1964 epidemic to 7 in 1983, and with rubella eliminated in the U.S. by 2004.
The introduction of routine use of the varicella vaccine in the United States in 1995, while significantly decreasing chickenpox in children, has caused an increase in the incidence of herpes zoster (shingles) in adults, due to loss of periodic boosting from exposure to chickenpox (see epidemiology of herpes zoster for references and discussion). As herpes zoster is generally more serious than chickenpox, there has been concern about this result. Since 2007, the varicella vaccine (rather, a larger dose) is now also recommended in the US for adults over 60, counteracting this. Further, as chickenpox vaccination becomes more widespread and the population ages, incidence of both chickenpox and herpes zoster may fall significantly. Nevertheless, at least initially, studies have shown some harmful results (increases in shingles) as a result of incomplete vaccination in the initial stage.

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