Type I vaccine-derived polioviruses in China from 1995 to 2019

A nation-wide case surveillance was conducted in China since 1995 for the objective of identifying acute flaccid paralysis (AFP) in children so that potential wild polioviruses and vaccine-derived poliovirus (VDPV) could be identified on time. Two outbreaks associated with type I circulating VDPVs, eight native independent type I ambiguous VDPVs (aVDPV), and one imported aVDPV were identified during the AFP case surveillance in China from 1995 to 2019. The VP1 coding region of the Chinese type I VDPVs differed from the polio vaccine strain by 1.00%–3.75% (9–34 substitutions in 906 nucleotides). Most of the Chinese type I VDPV strains shared 4 amino acid substitutions in the neutralizing antigenic (NAg) sites: 3 located at the BC loop, which formed the NAg site 1, and another at NAg site 3a. All of the Chinese type I VDPVs identified during the AFP case surveillance were young VDPVs, which indicated a limited viral replication resulted from the administration of the initiating oral polio vaccine (OPV) dose. VDPVs can emerge and spread in isolated communities with immunity gaps and the circulation ceases following a mass immunization with OPV. As such, high-quality surveillance permitted very early detection and response and it played a key role in stalling the widespread circulation of the emergent cVDPV strains in China.