Epidemiology and Risk Predictions

Statistical models can be very powerful in extracting information from extensive data sets, in summarizing trends, and in predicting risks. In our study of poliovirus epidemiology, we construct descriptive statistics and predictive risk models. With the first, we can describe patterns in the virus movement, study the herd immunity threshold, and study the apparent transmissibility of the virus across different age groups. With the second, we put together these and other insights into models to predict, among others, cVDPV emergence, and the propagation of WPV.

DESCRIPTIVE STATISTICS

  • Surveillance Sensitivity Assessment
  • Phylogeography

PREDICTIVE RISK MODELS

  • cVDPV RIsk Mitigation
  • Regional Outbreak Risk Mitigation
  • Sub-national Prioritization

DESCRIPTIVE STATISTICS

Surveillance Sensitivity Assessment

Disease surveillance efforts are key for timely identification of polio outbreaks and initiation of outbreak response. Historically, surveillance of cases of acute flaccid paralysis (AFP) from both polio and non-polio sources has been of central interest and is now increasingly joined by environmental surveillance of poliovirus in select sampling locations. Statistical analysis of AFP surveillance indicators can help identify areas of suspect or deteriorating surveillance while not overreacting to randomness in the observations. Probability models can aid in the assessing the potential sensitivity of environmental surveillance implementations.

Phylogeography

Polio doesn't infect every child in a country with equal probability. There are complex spatial patterns that are determined by heterogeneities in immunity, behavior, and social structure. Phylogeography offers a stastical tool to better understand spatial processes by combining case data with genetic information about how cases are related to each other by unobserved chains of transmission. We work with polio phylogenetic data to try to identify improved predictive models of spatial transmission and are interested in tree summary statistics as data for approximate Bayesian model fitting of heterogeneous models.

PREDICTIVE RISK MODELS

cVDPV RIsk Mitigation

The rare event of genetic reversion of oral polio vaccine (OPV) to strains with wild-like neurovirulence motivates the long term strategic policy of global OPV cessation. The first OPV to be ceased will be type 2; due to wildtype eradication, type 2 polio only exists in vaccine-derived forms and is the most common of all VDPV's. However, after cessation of OPV2 in routine immunization and SIA's, latent circulating VDPV may emerge into outbreaks. We are actively studying the conditions under which VDPV's emerge in order to quantify the risk in various countries of interest. Risk quantification can be used to aid in optimizing and directing risk mitigation efforts, such as the number and location of pre-cessation tOPV SIA's.

Regional Outbreak Risk Mitigation

Supplementary immunization activities have been a central strategy in polio eradication efforts. However, countries in endemic or outbreak regions are at risk of importing poliovirus. Thus reducing or maintaining low susceptibility with SIA's aids in minimizing or eliminating the potential for outbreaks from such importation. We contribute to risk assessment activities in support of global SIA planning by considering indicators of susceptibility and the potential for exposure to poliovirus.

Sub-national Risk and Resource Prioritization

To maximize the impact of different targeted interventions, is important that country programs target the areas (districts or sub-districts) at greatest risk for future wild or vaccine-derived poliovirus circulation. To do this, we have developed a set of spatio-temporal statistical models that--to a high degree of accuracy--predict areas at risk for polio transmission in six months into the future. We have developed models for Nigeria and Pakistan, and we are currently developing risk assessment type models for other countries in the middle east. See Publications for an example of this work.