Epidemiology and control of polioviruses in Nigeria

Abstract

Although the World Health Assembly resolved to eradicate wild poliovirus (WPV) by 2000, four countries have yet to interrupt transmission of WPVs. As of 2009, Nigeria has reported the largest burden of poliovirus of any single country for 5 of the last 6 years and been responsible for the largest outbreak of circulating vaccine-derived poliovirus (cVDPV) to date. With surveillance data on 36,410 acute flaccid paralysis cases from 2001-2009 in Nigeria, I use case-control methods to estimate vaccine efficacies of trivalent and monovalent oral polio vaccines (tOPV and mOPVs, respectively) against paralysis by all types of poliovirus circulating in Nigeria (serotypes 1 and 3 WPV and serotype 2 cVDPV (cCDPV2)). I show that mOPVs are substantially more efficacious than tOPV against serotype 1 and 3 and that tOPV is substantially more effective against cVDPV2 than against serotypes 1 and 3 WPVs. i use these results to investigate temporal and spatial variation in vaccine-induced immunity. I also estimate vaccine coverage and thus demonstrate that polioviruses persist in northern Nigeria as a result of low vaccine coverage and not due to any geographical variation in vacine efficacy. I show that the clinical characteristics and attack rate of cVDPV2 are as would be expected of a WPV and therefore similar control methods should be used. Using a proportional hazards model, I demonstrate that the risk of reporting cVDPV2 is strongly localised and cVDPV2 can be controlled with well targeted use of tOPV whilst maintaining control of WPVs with mOPVs. Finally I use negative binomial regression to estimate the predictors of incidence by geographical state in Nigeria. I conclude that the prospects for elimination of all polioviruses in Nigeria are very good with targeted use of vaccines of the appropriate serotype and provided vaccine coverage can be improved in high incidence areas.