Implication of a High Risk of Type 2 Vaccine-Derived Poliovirus Emergence and Transmission after the Switch from tOPV to bOPV.

BACKGROUND: China implemented the globally synchronized switch from trivalent oral poliovirus vaccine (tOPV) to bivalent OPV (bOPV) and introduced one dose of inactivated poliovirus vaccine (IPV) on May 1st 2016. We assessed the impact of the switch on the immunity level against poliovirus, especially type 2.

METHODS: Children born between 2014 and 2017, who were brought to the hospitals in Urumqi city, Xinjiang Province in 2017 were enrolled and blood samples were collected to test for antibody titres against poliovirus. The comparison of seroprevalence between the children born before (pre-switch group) and after the switch (post-switch group) were performed to assess the impact of the switch on the immunity level against polio.

RESULTS: A total of 172 subjects were enrolled. The overall seroprevalence is 98.8%, 79.1%, and 98.3% for type1, 2 and 3, respectively. Seroprevalence for type 2 significantly decreased from 91.6% in pre-switch group to 67.4% in post-switch group, but no statistically significant change was observed for both type 1 and 3.

CONCLUSION: The switch from tOPV to bOPV can provide high levels immunity against type 1 and 3 but not against type 2, indicating a high risk of type 2 vaccine derived poliovirus (VDPV) emergence and transmission.