Hinman and colleagues (1988) but was modified for a mixed IPV-OPV schedule. The model has two major components: the number of cases of VAPP (immunodeficient, recipient, contact, and community acquired) and the number of susceptible individuals in the United States.
The assumption was made that under an OPV strategy, the number of cases of VAPP would continue at about the same rate as that reported by CDC for the period 1980 to 1992. A strategy of all IPV or no polio vaccine would result in no cases of VAPP, under the assumption that IPV is always completely inactivated. With the parental choice approach, the number of cases of VAPP would range somewhere between no cases and that resulting from the all-OPV vaccine strategy, depending on what proportion of parents chose which vaccine. No data on this possibility are available for the United States. For an IPV-OPV strategy, the assumptions are more complex. Cases among immunodeficient individuals would be estimated under the assumption that IPV has no effect on the probability of VAPP for those with immunodeficiencies and that such a deficiency would not be detected at the time of OPV receipt. For immunocompetent recipients, it is necessary to know how IPV can protect against VAPP following subsequent OPV doses. The model uses serum antibody titers. Implicit in the assumption is that the reason most cases of VAPP occur after the first dose of OPV is that those who are immunized with at least one dose of OPV (and do not get VAPP) are protected for the succeeding doses of OPV. Factors in predicting the number of contact cases of VAPP include the percentage of recipients shedding virus, the duration of excretion, the amount of virus excreted, and the characteristics of the virus excreted.
The key parameters in estimating the number of susceptible individuals are vaccine coverage and rate of exposure to vaccine virus. The model treats immunization as an event occurring during the first year of life in a portion of the population. Some people are not protected by the vaccine because of vaccine failures or host characteristics. Those who are not so immunized are susceptible to polio. In succeeding years, susceptible individuals may come into contact with vaccine virus, resulting in either immunization, VAPP, or continued susceptibility. The model currently takes each yearly cohort to age 60.