Rapid Disappearance of Poliovirus Type 2 Immunity in Young Children Following Withdrawal of Oral Poliovirus Type 2 Containing Vaccine in Vietnam.

BACKGROUND: Due to global shortage of inactivated poliovirus vaccine and global withdrawal of poliovirus type-2 (PV2) containing oral vaccine in May 2016, Vietnam has not used any PV2-containing vaccine between May 2016-October 2018. We assessed population immunity gap to PV2.

METHODS: A cross-sectional survey in children 1-18 months of age was carried out in January 2018: one blood sample was obtained and analysed for the presence of poliovirus neutralizing antibodies. In children with detectable anti-PV2 antibodies, a second blood sample was obtained and analysed four months later to distinguish between passive (maternally-derived) and active (induced by secondary transmission or vaccination) immunity.

RESULTS: Analysable sera were obtained from 1,106/1,110 enrolled children. Seroprevalence of PV2 antibodies was 87/368 (23.6%) among 1-7-month-old; 27/471 (5.7%) in the 8-15-month-old; and 19/267 (7.1%) in the 16-18-month-old. Seroprevalence declined with age in the 1-7 month-old group, and in children 8-18 months it remained without significant change by age. Four months later, 11/87 (14%), 9/27 (32%), and 12/19 (37%) remained seropositive in the 1-7, 8-15, and 16-18-month groups, respectively.

INTERPRETATION: We found declining immunity to PV2, suggesting that Vietnam is at risk for an outbreak of type 2 vaccine-derived poliovirus in case of importation or new emergence.