The number of susceptible individuals for the OPV and IPV-OPV options can be calculated directly from the model. For IPV, the number of susceptible individuals is derived by adding the number not vaccinated to the number vaccinated but not immunized; this is dependent on the coverage level. For the no polio vaccine option, the number of susceptible individuals is considered the total population of all unvaccinated cohorts.

Major questions with respect to estimating the risk of VAPP through existing modeling techniques remain unanswered:

• Has there been a recent decline in immunocompetent individuals among cases of VAPP?

• What protection against VAPP is given by circulating antibodies?

  • What antibody level is required?

  • Is this host-related?

• What are the characteristics of shed virus?

  • Are there product differences?

  • Do studies on revertants conflict?

Questions with respect to predicting the number of susceptible individuals also remain:

• How would immunization coverage rates change with a more complicated schedule?

• What would be the coverage rates for OPV under a strategy with greater use of IPV?

• What is the “true” exposure rate to the vaccine strain of virus (including both vaccinees and contacts)?

Several participants suggested that this modeling approach could be promising but that before publication or wide-spread dissemination, the approach should be circulated to a variety of experts for further input, particularly regarding the assumptions that have been made.