[Assessment of the potential impact in Italy of extensive varicella vaccination programs based on a mathematical...

A mathematical model has been developed in order to assess the effect of extended programs of varicella vaccination on the epidemiology of the disease in Italy. The effect of different vaccination options have been estimated by the change in incidence of the disease and age distribution of cases over a short and long period of time. The developed mathematical model reproduces chickenpox transmission and immunisation; five strategies different for target age of vaccination and/or proportion of vaccinated subjects have been considered. In all scenarios the model pointed out an initial decrease of case frequency observed in the first 3-5 years, followed by a series of epidemic peaks, variable in number and size by vaccination strategy. Moreover, as the number of cases among infants decreases, the number of cases among adults increases. Such event is minimised only by very high vaccination coverage (80% in the first year of life and 50% at 12 years of age). Extensive programmes of vaccination against chickenpox must reach a high coverage as soon as possible in order to avoid undesirable effects that may move forward the age of cases and therefore should be offered to target age groups easy to reach.