Future Ramifications of Age-Dependent Immunity Levels for Measles: Explorations in an Individual-Based Model

Abstract

When a high population immunity already exists for a disease, heterogeneities, such as social contact behavior and preventive behavior, become more important to understand the spread of this disease. Individual-based models are suited to investigate the effects of these heterogeneities. Measles is a disease for which, in many regions, high population immunity exists. However, different levels of immunity are observed for different age groups. For example, the generation born between 1985 and 1995 in Flanders is incompletely vaccinated, and thus has a higher level of susceptibility. As time progresses, this peak in susceptibility will shift to an older age category. Simultaneously, susceptibility will increase due to the waning of vaccine-induced immunity. Older generations, with a high degree of natural immunity, will, on the other hand, eventually disappear from the population. Using an individual-based model, we investigate the impact of changing age-dependent immunity levels (projected for Flanders, for years 2013 to 2040) on the risk for measles outbreaks. We find that, as time progresses, the risk for measles outbreaks increases, and outbreaks tend to be larger. As such, it is important to not only consider infants when designing strategies for measles elimination, but to also take other age categories into account.