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对１９９４年中国分离的１３株脊髓灰质炎Ⅱ型疫苗相关株进行了ＰＣＲ－ＲＦＬＰ分析，发现７株为重组病毒，毒力较疫苗株有回复。在Ⅱ型脊髓灰质炎病毒基因序列上，对于神经毒力有重要影响的第４８１位核苷酸发生了突变（Ｃ→Ｇ），另一个被视为重要位点的第２９０８位核苷酸无一发生变化，反而在第２９０９位核苷发生了高频率的点突变，意味着２９０９位点在中国Ⅱ型疫苗相关株的自然变异中可能起着重要作用。根据转基因小鼠实验和温度敏感实验（Ｒｃｔ－ｍａｒｋｅｒ），脊髓灰质炎病毒的神经毒力大小并不与温度敏感实验所显示的毒力特征呈正相关性。
PCR-RFLP analysis of 13 strains of poliovirus type Ⅱ vaccine isolated in 1994 in China showed that 7 strains were recombinant viruses, and the virulence of these strains was higher than that of the vaccine strains. In the type II poliovirus gene sequence, mutation at nucleotide 481 (C → G), which has an important effect on neurovirulence, was observed and the other was considered as the 2908th nucleotide of the important site On the contrary, the high frequency point mutation occurred at the 2909th nucleoside, which means that the 2909 site may play an important role in the natural variation of the Chinese type II vaccine-associated strains. According to the transgenic mouse experiment and temperature-sensitive test (Rct-marker), the size of the virulence of poliovirus is not positively correlated with the virulence characteristics shown by temperature-sensitive experiments.