Source: Masters Abstracts International, Volume: 57-05.

Adviser: Virginia E. Pitzer.

Introduction: The current view of vaccine effectiveness does not capture the true nature of vaccine behavior for a full range of communicable diseases. In order to observe a more inclusive and realistic behavior, it is necessary to create a broader mathematical model that encompasses different biological features to explore their impact on the overall effectiveness of vaccination.

Methods: An SIRS model was developed and modified to test the effect of 7 different parameters on the overall effectiveness of vaccination, as measured by the percent reduction in cases over 10 years following vaccine introduction. The parameters varied (and corresponding ranges) were: R 0 (2-20; R0 = beta/gamma.), duration of immunity (DIM; 1-10 years), relative risk of re-infection (RR; 0-1), relative infectiousness of subsequent/breakthrough infections (RI; 0.1-1), the level of vaccine protection (VP; "all-or-nothing": 0.5-1; "leaky": 0-1), relative duration of vaccine-induced immunity compared to natural immunity (RD; 1-10), duration of infectiousness (DIF; 1-10 weeks), and vaccine coverage (VC; 0%, 30%, 50%, 60% and 80%). Three scenarios were explored: 1) an SIR-type model (relative risk of reinfection fixed to 0), 2) a "flu-like" SIRS model (R 0=1.3, DIM=1), and 3) an imperfectly immunizing childhood infection model (R0=6, DIM=5).

Results: When the 7 parameters were varied, common trends were detected across the three scenarios. Generally, with an increase in R0, RR, RI, and DIF, the percent reduction in cases over 10 years decreases. Also, with an increase in VP (leaky and all-or-nothing), DIM, and RD, the percent reduction in cases over 10 years increases.

Conclusion: Though a definite conclusion whether one parameter is more important than other parameters cannot be determined due to the model's multidimensional nature, we showed that some parameters---RI and R 0, in particular---have a greater impact on overall vaccine effectiveness than others. The framework we developed is versatile and can be applied and fitted to explore the overall vaccine effectiveness for a wide range of vaccine-preventable diseases.