In mathematical modelling in epidemiology, 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 program 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. This is called an epidemiological shift. In a disease like rubella, which has 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.
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, with a large drop in CRS incidence the early 1980s and with rubella eliminated in the U.S. by 2004.