Rapid Disappearance Of Poliovirus Type 2 (Pv2) Immunity In Young Children Following Withdrawal Of Oral Pv2-Containing Vaccine In Vietnam

Background Due to global shortage of inactivated poliovirus vaccine and withdrawal of oral vaccine containing poliovirus type 2 (PV2), a PV2-containing vaccine was not used in Vietnam May 2016 to October 2018. We assessed the population immunity gap to PV2.Methods A cross-sectional survey in children aged 1-18 months was carried out in January 2018. One blood sample per child was analyzed for presence of poliovirus neutralizing antibodies. In children with detectable anti-PV2 antibodies, a second sample was analyzed 4 months later to distinguish between passive (maternally derived) and active (induced by secondary transmission or vaccination) immunity.Results Sera were obtained from 1106/1110 children. Seroprevalence of PV2 antibodies was 87/368 (23.6%) at age 1-7 months, 27/471 (5.7%) at 8-15 months, and 19/267 (7.1%) at 16-18 months. Seroprevalence declined with age in the 1-7 months group; in the 8-18 months group there was no significant change with age. Four months later, 11/87 (14%), 9/27 (32%), and 12/19 (37%) remained seropositive in 1-7, 8-15, and 16-18 months age groups, respectively.Conclusions We found declining immunity to PV2, suggesting Vietnam is at risk for an outbreak of type 2 vaccine-derived poliovirus following virus importation or new emergence.Declining immunity to poliovirus type 2 after withdrawal of type 2 poliovirus oral vaccine was documented in Vietnam. This immunity gap poses risk of widespread outbreak of circulating vaccine-derived poliovirus type 2 in case of virus importation from epidemic areas.