Pre-empting poliovirus: identifying countries at risk of outbreaks
Reinfection of previously polio-free countries and the resulting outbreak response activities cost the Global Polio Eradication Initiative (GPEI) close to US$1 billion during 2003-2009. Those countries most at risk of an outbreak have historically been close to remaining endemic areas or had significant gaps in immunization coverage. However, historical patterns are not always a good indicator for future risk. In 2008-2009, polio cases were reported across Africa in a series of outbreaks on an unprecedented scale1. Many of these countries experienced their first polio outbreak after stopping indigenous wild poliovirus transmission.
To understand the factors that put a country at risk of a polio outbreak a team from the World Health Organization (WHO) and Imperial College London studied polio surveillance and demographic information, including movement data from across Africa. Regression models showed that poor routine immunization coverage with three doses of oral polio vaccine (OPV), high levels of exposure to poliovirus from Nigeria through population movement and a young population were all independently associated with a larger number of polio outbreaks. Periodic supplementary immunization activities were implemented in some countries to protect against outbreaks, but were of variable quality over the period of analysis and no strong protective effect was observed.
Country-specific forecasts of outbreak risk six months ahead of time can be produced using routinely reported data on the risk factors that were identified by the regression analysis. These forecasts were found to have a reasonably good predictive ability and identified countries newly at risk in advance of an outbreak, and accurately described the increased number of outbreaks in 2008-2009 and show a reduced risk in 2010 as a result of the recent decline in the number of polio cases reported in Nigeria. Continued use of these forecasts will help optimize resource allocation in Africa to minimize the number of polio outbreaks during 2010-2012.
1Resurgence of wild poliovirus types 1 and 3 in 15 African countries, January 2008-March 2009. Weekly Epidemiological Record. No. 16, 2009, 84, 133-140.