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The potential risk of increasing the burden of infection after immunisation originates from the fact that a vaccination programme tends to increases the average age at infection of the non-immunised.

This has been shown both by mathematical models and by experience in many countries and for a number of diseases.

The figure shows how the average age at infection changes with vaccination coverage for two different average ages at which children are vaccinated. (It is based on a simple model assuming homogeneous mixing of a population with an age structure of the type of developed countries.)

This effect can also be intuitively understood. As vaccination coverage increases the spread of the infection decreases and the chances of an unimmunised individual to get infected also decrease, on average. For the same chances to be accumulated as prior to vaccination, more time has to elapse, on average; therefore, there is an increase in the average age at infection.

The younger the age at which vaccination is given, the larger the effect (because more children would avoid infection).

If vaccination coverage is below a certain threshold, this (expected) relative increase of disease occurrence in older age-groups may be accompanied by an absolute increase of diseased adults; and, if the clinical consequences of the disease are more severe in adults, the overall disease burden in the population will be larger.

(Source of figure: Anderson R, May R. Infectious diseases of humans. Oxford: OUP; 1991. p.93.)