November 1, 2007


Vaccine-derived Polio Outbreak in Nigeria
By D.A. Henderson, MD, MPH, FIDSA
Center for Biosecurity, University of Pittsburgh Medical Center


The largest outbreak to date of poliomyelitis caused by vaccine-derived polioviruses (cVDPV) has struck Nigeria. Ninety-three cases have been reported from October 2005 to August 2007, with two additional related cases in neighboring Niger. Current Centers for Disease Control and Prevention (CDC) reports show no further cases since August.

polio virionsThe isolates derived from mutated type 2 oral vaccine strains that had combined with indigenous Coxsackie-type enteroviruses resulting in recombinant virus forms.  Of the 95 isolates, 71 showed a typical divergence from the genome sequence of the vaccine strain; the 24 others exhibited a less marked divergence but were clearly derived from the type 2 vaccine. 

cVDPV is known to evolve, in rare cases, from any of the three live, attenuated viruses used in oral polio vaccine.  During the 20-month period in this most recent report, CDC identified only six other VDPV recombinant isolates–two of type 3 from Cambodia and four from Myanmar.

In 2003, false rumors that the vaccine was unsafe had shut down vaccination campaigns in northern Nigeria, which likely contributed to the outbreak of cVDVP. However, the campaigns have been able to be restarted and have been intensified using monovalent strains. As of September 30, Nigeria had reported 220 cases of non-VDVP polio cases in 2007, all cases being either type 1 or type 3.  This is substantially fewer cases than the 921 cases reported during this period last year.

For more information, see the Sept. 28 Morbidity and Mortality Weekly Report.

Also see www.polioeradication.org.

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