ISSUE: The introduction of novel vaccines for infectious diseases leads to decreases in disease among vaccinated individuals due to direct effects. Changes in disease may also occur in unvaccinated individuals due to indirect effects, such as herd immunity and serotype replacement. On one hand, indirect effects are a key feature of infectious disease biology and health economics, extending the value of a vaccine or treatment beyond the individuals receiving it. On the other hand, indirect effects are difficult to quantify and incorporate into health economic models. This panel will examine the public health effects due to indirect effects and discuss the implications of their inclusion or exclusion from health economic models.

OVERVIEW: Historically, health economic models have primarily quantified direct effects, but there is increasing evidence of herd immunity and serotype replacement following vaccine introductions for different pathogens. The hurdles lie in the uncertainty of these effects and the intricate population and disease dynamics driving them, complicating the validation and assessment of modeling outcomes. From the public health, modeling, and health technology assessment perspectives, this panel will debate the approaches to including indirect effects in health economic models of infectious disease and the implications in medical decision-making. Dr. Huang will moderate the session and provide an overview of recent research and challenges and approaches in modeling indirect effects. Dr. Orenstein will share observations of indirect effects and other phenomena associated with vaccines from the public health perspective. Dr. Oidtman will discuss effects like herd immunity and serotype replacement in the context of health economic modeling and model validation approaches. Dr. Drummond will discuss how uncertainties associated with indirect effects manifest when demonstrating value to decision makers while emphasis on future research and validation efforts. The attendees will be invited to share their views and pose questions to the panelists.