CIRCULATION OF TYPE-1 WILD POLIOVIRUS IN NORTHERN VIETNAM DURING 1991-1994

From 1991 to 1994, 143 polioviruses were isolated from patients with a cute flaccid paralysis in northern Vietnam. Of these 143 isolates, 133 were type 1 and five of each were type 2 and type 3. These isolates w ere intratypically differentiated by polymerase chain reaction-restric tion fragment length polymorphism (PCR-RFLP). Of the 133 type 1 isolat es, 113 were wild strains and only 20 isolates were of Sabin vaccine-l ike strains. These 113 isolates were divided into seven groups by PCR- RFLP patterns and were also classified into three genomic groups by nu cleotide sequencing and phylogenetic analysis. One of the three genomi c groups accounted for the majority (75) of these isolates. The isolat es belonging to the predominant group were found during 1991-1993. The second group, which included 17 isolates, were detected only in 1993. These isolates had genome sequence similar to isolates in southern Vi etnam in the same year. The third group of isolates were detected only in 1991 and were considered to be Mahoney-like wild strains. All isol ates examined were different from those obtained in other countries. I n 1994, however, no wild-type polioviruses were isolated in our study. These results reveal that three unique strains were circulating in no rthern Vietnam in recent years, and indicate that the incidence of pol iomyelitis due to wild poliovirus is decreasing.