Perverse effects of vaccination programmes manifest themselves when insufficient numbers of susceptibles are vaccinated to reach the 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 poupulation; 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. These are the perverse effects.

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