Wild poliovirus circulation among healthy children immunized with oral polio vaccine in Antananarivo, Madagascar

From July 1995 to December 1996, 3185 stool specimens from healthy children aged 6-59 months attending 6 dispensaries in the Antananarivo area were examined for paliovirus. The children had been routinely immunized according to the Expanded Programme on Immunization (EPI) schedule and received the last dose of oral Folio vaccine (OPV) more than 1 month before stool collection. 99.4% of the children were immunized with at least 3 doses of OPV. HEp-2 cell culture revealed virus infections in 192 stools (6.0%), including 9 poliovirus (0.3%) and 183 nonpolio enterovirus isolates (5.7%). Infections occurred throughout the year, but incidence was higher during the hot and rainy season (P = 0.01). Using a neutralization test with monoclonal antibodies and PCR-RFLP in two genomic regions coding for the VP1 capsid and RNA polymerase, 4 wild polioviruses (3 type 1 and 1 type 3) and 5 vaccine-related polioviruses (2 Sabin 1-like variants, 1 Sabin 2-like and 2 Sabin 3-like) strains were identified. The wild polioviruses were isolated at the beginning and the end of the dry season. Similar RFLP patterns were observed for the 3 wild type 1 polioviruses. Comparison of partial genomic sequences in the VP1/2 A region of 1 of the wild type 1 isolates with 2 wild type strains isolated in Antananarivo in 1992 and 1993 showed a divergence of at least 10% between the strains, suggesting at least two different pathways of transmission during this period. Our findings demonstrate that immunization with 3 doses of OPV did nor prevent intestinal carriage of wild poliovirus strains, and that there is a risk of wild poliovirus transmission to susceptible children in the area. Multiple strategies are required to improve immunization coverage in Madagascar.