a1 Division of Epidemiology and Biostatistics, European Institute of Oncology Via Ripamonti 435, 20141 Milano, Italy
a2 Dipartimento di Statistica e Matematica Applicata all' Economia, Università di Pisa Via Ridolfi 10, 5612 Pisa, Italy
a3 Dipartimento di Scienze Economiche e Metodi Quantitativi, Università del Piemonte Orientale “A. Avogadro”, Via Perrone 18, 28100 Novara, Italy
Simple epidemiological models with information dependent vaccination functions can generate sustained oscillations via Hopf bifurcation of the endemic state. The onset of these oscillations depend on the shape of the vaccination function. A “global” approach is used to characterize the instability condition and identify classes of functions that always lead to stability/instability. The analysis allows the identification of an analytically determined “threshold vaccination function” having a simple interpretation: coverage functions lying always above the threshold always lead to oscillations, whereas coverage functions always below never lead to instability.
(Online publication June 15 2008)
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