Violence, Insecurity, and the Risk of Polio: A Systematic Analysis

"Are violence and instability on their own predictive of whether a country will have future polio cases? Do violence and instability indicate risk of poliovirus after adjusting for available measures of a country's susceptibility and poliovirus exposure?"

Countries with high scores on the Global Peace Index (GPI), a composite measure of peace using 23 qualitative and quantitative indicators, align roughly with countries that have had poliovirus cases over the last 10 years (e.g., Afghanistan, Pakistan, and Northern Nigeria). This paper examines how physical and socio-political insecurity influences the risk of polio spread, and whether consideration of such factors augments a purely virologic/immunologic understanding of risks posed to eradication. That is, it extends previous investigations by evaluating the relationship between incidence of polio and instability in the context of a broader epidemiological model of the risk of disease, which includes infant mortality rate, routine immunisation coverage, history of polio, and proximity to previous poliovirus circulation.

Using logistic regression models and public data sources, the researchers evaluate the relationship between measures of violence and instability and the location of poliomyelitis cases at the country level, both individually and after controlling for more proximal determinants of disease, such as nearby circulating poliovirus and vaccination rates. They limited their analysis to countries reporting to the African, Eastern Mediterranean, and South East Asian regional offices of the World Health Organization (WHO) where polio was endemic during the period considered (2006-2015).

In short:

• In Model 1, the researchers used a simple logistic regression model to test each variable for its association with future polio cases. Table 2 in the paper displays results; in short, all variables in Model 1, with the exception of intentional homicide rate and migrant stock, show strong association with poliovirus cases in the subsequent year. Notably, increases in a country's Fragile States Index (FSI) and GPI were associated with the occurrence of poliovirus cases in the subsequent year. The association between GPI and polio can be seen in the map in Figure 1 in the paper (and above), supporting the observation that recent polio cases have occurred largely in countries with security concerns.
• Model 2 looks at whether each variable adds to a basic model of risk using the following intrinsic poliovirus epidemiology variables: (i) the presence of wild poliovirus (WPV) or vaccine derived poliovirus (VDPV) cases in the previous year, (ii) the presence of a WPV/VDPV case in a neighbouring country in the previous year, (iii) coverage of the third dose of diphtheria-tetanus-pertussis vaccine (DTP3), and (iv) population size. The researchers found that after adjusting for intrinsic polio variables, most variables remain associated with polio cases in the subsequent year, although measures of direct violence (number of terrorist events and intentional homicide rate) do not. Both FSI and GPI remain highly predictive of future polio cases, with increases in FSI and GPI again being associated with increased risk.
• Model 3 examines whether indicators of violence and instability add to a model based on an expanded set of covariates: those in Model 2 as well as infant mortality rate, migrant stock, and access to an improved water source. The researchers found that all variables except intentional homicides and population appear associated with WPV and VDPV in subsequent years.

Figure 2 in the paper illustrates the predicted risk of a case in 2013, comparing a model with only intrinsic polio variables described above, and that same model including all variables related to violence and instability (FSI, GPI, terrorist events, and homicide rate). In the highest-risk countries (Afghanistan, Pakistan, Somalia, Nigeria, Chad, and Niger), polio epidemiology alone was enough to predict occurrence of cases in 2013, a year with numerous outbreaks. However, inclusion of factors related to violence increases the accuracy of these predictions.

Thus, the findings from this study suggest that political instability and violence are related to risk of wild and vaccine-derived polio cases in part through interaction with health systems and traditional epidemiological risk factors, and further, that violence and instability capture risk that is un-measured in a model based on traditional risk factors.

The researchers point out that in this study and in risk models in general, the interpretation depends on the quality and granularity of available data. Also, the large number of variables comprising the GPI and FSI make it challenging to isolate the most significant contributing factors to the association between violence and polio, requiring further research into the nuances of political instability. Furthermore, they stress, while this work highlights the importance of risk assessment in conflict-affected countries, it should be complemented by research into how health programmes can adapt and operate within these environments.

In conclusion: "It is important to document the key lessons learned from global polio eradication, including those related to humanitarian access, negotiation, and innovative delivery approaches in complex settings such as Afghanistan, Somalia and Northern Syria. The broader global health and security communities need to engage more deeply with one another to address the challenge of delivering health care in conflict affected areas."