Investigation of a Type 2 Vaccine-Derived Polio Virus Event in Sichuan Province, China 2019

  • Healthcare related infections
  • Vaccine preventable diseases
  • Other
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Background:
In 2016, type 2 poliovirus was withdrawn globally from trivalent live attenuated polio vaccine (OPV) to stop generation of type 2 vaccine-derived polioviruses (VDPV2). China subsequently changed routine immunization to one dose of inactivate polio vaccine (IPV) followed by three doses of bivalent (I+III) OPV. In June 2019, a VDPV2 was detected from an acute flaccid paralysis (AFP) case in Sichuan province. We investigated the VDPV2 event and monitored the response.

Methods:
We reviewed AFP surveillance data and searched for additional AFP cases; collected stool specimens from the case, contacts, and local communities; conducted environmental surveillance (ES) in the county and high-risk prefectures; and conducted a coverage survey. The affected prefecture conducted two non-selective Sabin-strain-IPV campaigns, and surrounding prefectures conducted catch-up IPV campaigns.

Results:
The VDPV2 had 28 mutations from the vaccine strain, and had 9 mutations in common with a 13-mutation VDPV2 isolated in Xinjiang province ES in 2018. Three VDPV2s were detected in healthy Sichuan children - two from contacts of the initial case and one from another local child. Prior to the event, polio vaccine coverage was 65% among under-5-year-olds in surrounding townships based on convenience sampling. The AFP case rate was above 1 per 100 000; none of these AFP cases were polio. No type 2 polioviruses have been isolated through ES. All IPV campaign coverage rates were above 98%.

Conclusions:
This VDPV2 outbreak followed cessation of OPV2; no transmission source was identified. The VDPV likely circulated for three years, enabled by weak routine immunization in the county. The IPV campaigns appear to have prevented further VDPV2 spread, however, it is uncertain whether an IPV-only strategy will completely stop transmission. We recommend enhancing AFP and environmental surveillance and introducing a second IPV dose into routine immunization.

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