Reemergence of recombinant vaccine-derived poliovirus outbreak in Madagascar.

Abstract : BACKGROUND: After the 2001-2002 poliomyelitis outbreak due to recombinant vaccine-derived polioviruses (VDPVs) in the Toliara province of Madagascar, another outbreak reoccurred in the same province in 2005. METHODS: We conducted epidemiological and virological investigations for each polio case patient and for their contacts. RESULTS: From May to August 2005, a total of 5 cases of acute flaccid paralysis were reported among unvaccinated or partially vaccinated children 2-3 years old. Type-3 or type-2 VDPV was isolated from case patients and from healthy contacts. These strains were classified into 4 recombinant lineages that showed complex mosaic genomic structures originating from different vaccine strain serotypes and probably from human enterovirus C (HEV-C) species. Genetic relatedness could be observed among these 4 lineages. Vaccination coverage of the population was very low (<50%). CONCLUSIONS: The broad distribution of VDPVs in the province and their close genetic relationship indicate intense and rapid cocirculation and coevolution of the vaccine strains and of their related HEV-C strains. The occurrence of an outbreak due to VDPV 3 years after a previous outbreak indicates that a short period with low vaccination coverage is enough to create favorable conditions for the emergence of VDPV in this setting.