Disease Transmission Dynamics

Disease modeling plays a crucial role in evaluating what impact disease risk factors and control measures will have over time. In certain cases, simple models can be sufficient in describing population-wide dynamics, and the most salient features of polio epidemiology. In other cases, we dive into the details of the immunology, the transmission, and the population structure in order to capture what will make the models most realistic and complete with respect to the question being studied. In this section, we present both models using both simple and detailed disease models.

Simplified Disease Model

  • Northern Nigeria Model
  • cVDPV Emergence

Detailed Disease Model

  • Immune Response Model
  • Critical Community Size
  • Expanded Age Group Vaccination
  • Tajikistan Outbreak Model

SIMPLIFIED DISEASE MODEL

Northern Nigeria Model 

As global polio eradication draws near – the current eradication strategic plan expects wild transmission to be interrupted in 2015, with global certification in 2018 – models describing the dynamics of transmission in the remaining endemic countries will play an important role in understanding the conditions required for local elimination. Northern Nigeria is one of three remaining endemic countries, and the eradication of WPV1 and prevention of cVDPV2 circulation in the future represent programmatic challenges. Our model of northern Nigeria aims to provide insight into the critical features of disease transmission within the region and provide actionable outputs for local program planning. Simple models of disease transmission, ignoring heterogeneity and spatial dispersion of populations, can generally serve very well to describe outbreak scenarios or well-established endemic diseases. However, in recent years, polio incidence in northern Nigeria has declined dramatically and become substantially more localized, and so we also seek to describe the increasingly important effects of spatial heterogeneity in transmission and accessibility to vaccination campaigns, to understand the necessary conditions for country-wide eradication.

DETAILED DISEASE MODEL

Immune Response Model

To examine forces that drive vaccination policy to eradicate wild- and vaccine-derived poliovirus, and to focus on the efficacy of vaccines to support decision-making and further research, we collected existing literature on polio vaccine immunogenicity and protection, we conducted a meta-analysis of human immunity to polio infections using multiple non-linear regressions, and we built a database from a broad (but not exhaustive) set of polio vaccine studies (46 studies, >10000 subjects). The outcome was an immunological model representative of many different datasets. Parameters measured immunogenicity to both humoral and mucosal immune compartments for Salk and Sabin vaccines. The immunity model was more highly correlated with the data than a simpler per-dose efficacy model. The model offers new insights for immunization policy. We measured the mucosal immunogenicity of IPV to a precision that is useful in decision-making for end-game polio immunization policies.

Critical Community Size

Critical community size (CCS) may be generally defined as the minimum population size required to sustain transmission of an infectious disease in the absence of importation from outside the community. The CCS will depend upon the force of infection, the duration of the infectious period, birth rates, local within-community migration rates, vaccination rates, as well as seasonality in transmission. Beginning with single homogeneously mixed populations, we have worked to establish CCS for poliomyelitis as a function of transmission environment. We have subsequently expanded the work to investigate the effect of geographic heterogeneity on CCS, including communities comprised of multiple geographically distinct populations linked through migration.

Expanded Age Group

A priority of the Global Polio Eradication Initiative (GPEI) 2013-2018 strategic plan is to evaluate the potential impact on polio eradication resulting from expanding one or more Supplementary Immunization Activities (SIAs) to children beyond age five-years in polio endemic countries. It has been hypothesized that such expanded age group (EAG) campaigns could accelerate polio eradication by eliminating immunity gaps in older children that may have resulted from past periods of low vaccination coverage. Using an individual-based mathematical model, we quantified the impact of EAG campaigns in terms of probability of elimination, reduction in polio transmission and age stratified immunity levels. The model was specifically calibrated to seroprevalence data from a polio-endemic region: Zaria, Nigeria. We compared the impact of EAG campaigns, which depend only on age, to more targeted interventions which focus on reaching missed populations. We found that EAG campaigns would not significantly improve prospects for polio eradication. In contrast, expanding only two SIAs annually to target hard-to-reach populations at modest vaccination coverage led to a significant increase in elimination probability.