Characterization of a rare natural intertypic type 2/type 3 penta-recombinant vaccine-derived poliovirus isolated from a child with acute flaccid paralysis.

A type 2 vaccine-derived poliovirus (VDPV) (strain CHN1025), with a 1.1 % (10/903) difference from Sabin strain in the VP1 coding region, was isolated from a child with poliomyelitis caused by a poliovirus variant infection. The patient was from Shandong Province of China and developed acute flaccid paralysis in 1997. The child was infected with a rare and complicated penta-recombinant poliovirus with the uncommon genomic recombinant organization S2/S3/S1/S3/S1/S3. At least five successive rounds of recombination occurred in the VP1 capsid coding region and in the 2C, 3C (twice) and 3D(pol) non-capsid coding regions, respectively, during virus evolution. Strain CHN1025 had most of the characteristics of the type 2 vaccine strain; it had Sabin-specific epitopes, suggesting that the virus was antigenically indistinguishable from the Sabin 2 reference strain. Typical mutations in the 5'-untranslated region and VP1 associated with reversion to neurovirulence for Sabin 2 poliovirus were found, and the virus showed moderate neurovirulence in transgenic mice. A few nucleotide substitutions were located in the donor sequences, and two donor sequences contained no nucleotide substitutions, suggesting that these sequences were relatively new. The appearance of these mutations within approximately 192 days of at least five successive rounds of recombination events derived from a single ancestral infection illustrates the rapid emergence of new recombinants among VDPVs. This is the first report on the isolation of a type 2/type 3 poliovirus capsid recombinant with one of the five crossover sites located in the VP1 coding region.