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We examined four type 1 polioviruses isolated from the stools of patients with vaccine-associated paralytic poliomyelitis in China. All of these isolates were shown to be Sabin derived viruses by restriction fragment length polymorphism assay after polymerase chain reaction and by sequencing of the viral genome encoding the viral coat protein, VP1. However, the same analysis of the 3D coding region suggested that two of the four isolates had the sequence of wild type poliovirus in the tested region. Furthermore there were also point mutations in the 5' non-coding region. One was a single base change from U to C at nucleotide position 525, and the other three were from G to A at position 480. All the four strains were more neurovirulent that Sabin type 1 virus in transgenic mice with human poliovirus receptor gene. The data showed that the nucleotide positions of type 1 poliovirus which were identified to be in favor of the high neurovirulence were indeed changed during natural transmission, and suggested that the point mutation alone or a recombination of the vaccine type with wild type genome results in an acquisition of neurovirulence.

We examined the genomic variability of 26 wild-type 1 polioviruses isolated from 12 provinces in China from 1985-1993. The poliovirus genome region that encodes the N-terminal portion of the VP1 protein was amplified by the polymerase chain reaction (PCR). The PCR product of each isolate was analysed by restriction fragment length polymorphism (PCR-RFLP) and by direct sequencing. Several groups of viruses with different PCR-RFLP patterns were found to cause outbreaks in China during this period. Viruses with identical PCR-RFLP profiles were dominantly isolated in 11 provinces from 1991 to 1993 and were considered to be the major epidemic strain in China. On the other hand, in the GuangDong province, a different group was responsible for each year's outbreak, while in the XinJiang autonomous prefecture, a single strain was consistently isolated from 1990 to 1993. Understanding of the nationwide distribution of wild poliovirus is very important for interrupting its transmission during the final stage of the eradication program in China.