Investigation of an outbreak of type 3 wild poliovirus in Cote d'Ivoire in 2011

Abstract

Background: Côte d'Ivoire had no type 3 wild poliovirus (WPV3) outbreak since 1999. In 2011, the country was reinfected by WPV3. The objective of this study is to explore the reasons behind the occurrence of WPV3 outbreak in Côte d'Ivoire in 2011.

Method: Data on WPV3 cases were obtained from specific investigation records during the outbreak investigation. Data on AFP cases was obtained from AFP routine investigation forms. Information on polio immunization was extracted from National EPI databases and documents. Univariate analyses were done to summarize characteristics of AFP and WPV3 cases. Bivariate and multivariable analyses were done to compare polio cases to non-polio AFP cases. Finally, the proportion of non-vaccinated children in routine immunization from 2006 to 2010 was calculated.

Results: In the Côte d'Ivoire outbreak, from January to July 2011, 36 WPV3 cases were notified. The median age was two years old (interquartile range two to four years old). Among these cases, 29 had received less than three polio vaccine doses. The majority of WPV3 cases were living in precarious socio-economic conditions. Regarding vaccination status of AFP cases, the polio cases had a statistically significant higher risk to have less than three doses of polio vaccine compared to non-polio cases, OR: 16.9 [95% CI: 2.3 – 125.0]. More than 27% of the children of less than one year old who were expected to be vaccinated in the country were not vaccinated from 2006 to 2010.

Conclusion: Despite the limitations, this study shows that a country that has interrupted polio transmission for one type of poliovirus can still be at high risk for polio outbreaks of this same type following an importation. This can occur when routine immunization coverage is low, polio supplementary immunization activities are done with only the poliovirus vaccine against the circulating poliovirus, people live in precarious socio-economic conditions and AFP surveillance is poorly performed. Côte d'Ivoire experienced this outbreak as many of these conditions occurred together.