The AFP case was of a 4-year-old boy born in November 2014 in a remote village of Leibo county, Liangshan Prefecture, Sichuan Province (Figure 1). Illness onset was April 25, 2019, when both of his legs and his left arm were found to be paralyzed 7 days after symptoms first appeared. By vaccination record and the parents’ recall, the child had received tOPV in March 2015 and May of 2015, approximately one year prior to the switch from tOPV to bOPV.
Stool specimens were collected on May 17 and 18, 2019. The provincial CDC laboratory found a type 2 poliovirus; China CDC confirmed the VDPV2 and determined it had 28 nucleotide changes from the vaccine strain and shared 9 nucleotide changes with a VDPV2 isolated from sewage in Urumchi, Xinjiang Autonomous Region on April 18, 2018 (the Xinjiang VDPV2 had 13 nucleotide changes from the vaccine strain) (3). The child recovered without residual paralysis and was discharged from the hospital. His subsequent stool specimens were negative for VDPV2 (Table 1).
Sample AFP case Contact 1 Contact 2 Healthy child 1st Collection date 2019.5.7 2019.6.14 2019.6.27 2019.8.18 Isolation result PV type 2 PV type 2 PV type 2 PV type 2 Nucleotide changes* 28 27 33 27 2nd Collection date 2019.5.18 2019.6.27 2019.7.7 2019.9.18 Isolation result PV type 2 NPEV NPEV Negative Nucleotide changes 28 –† –† –† 3th Collection date 2019.6.17 2019.7.7 2019.7.15 2019.9.25 Isolation result NPEV NPEV Negative Negative 4th Collection date 2019.6.22 2019.7.15 2019.7.24 2019.10.9 Isolation result Negative Negative Negative Negative 5th Collection date 2019.6.30 2019.7.24 –§ 2019.10.18 Isolation result NPEV Negative –§ Negative 6th Collection date 2019.7.7 –§ –§ –§ Isolation result NPEV –§ –§ –§ Abreviation: PV=Poliovirus, NPEV=Non-polio enterovirus.
* Nucleotide changes from vaccine strain.
† Because no poliovirus was detected, no further comparisons were necessary.
§ Two negative results indicated that no further specimens and testing were needed.
Table 1. Stool sample results from the acute flaccid paralysis (AFP) case, his contacts, and one other child.
We tested stool specimens for poliovirus from children under 5 years old in the affected child’s neighborhood and village and the hospital that treated him. Among 160 healthy children investigated, specimens from 2 children who lived 4 kilometers from the residence of the AFP case were positive for VDPV2s—one with 27 nucleotide changes and the other with 33 changes. We detected another VDPV2, with 27 nucleotide changes, in a healthy child in Leibo county who lived 5.5 kilometers from the initial child during investigation of 300 other healthy children (Table 1).
We assessed polio vaccine coverage in the village and surrounding townships among 88 children 1–5 years old in the village and 164 children <6 years old in 4 surrounding townships. In the village, 53.4% (47/88) had no history of polio vaccination, 35.2% (31/88) received 1 dose, 6.8% (6/88) received 2 doses, 3.4% (3/88) received 3 doses, and 1.1% (1/88) received 4 doses. In the surrounding townships, 65.0% (106/164) received 3 or more doses of polio vaccine.
We conducted retrospective searches for AFP cases in Liangshan on June 12 and five surrounding prefectures on June 17. The provincial medical investigation team reviewed 31,631,487 in-patient and out-patient records from 778 hospitals, going back to May 1, 2016. Among the 279 AFP cases found, 10 had not been reported previously. These newly-identified AFP cases were reviewed by provincial polio experts, and polio was diagnostically excluded from all 10. AFP “zero case reporting”, which is requiring absence of cases to be reported every day to ensure complete reporting of any cases, was started on June 21 (August 19 in Aba) to enhance sensitivity of surveillance in Liangshan and its 6 surrounding prefectures.