Background: The endgame for polio eradication involves coordinated global cessation of oral poliovirus vaccine\n(OPV) with cessation of serotype 2 OPV (OPV2 cessation) implemented in late April and early May 2016 and\ncessation of serotypes 1 and 3 OPV (OPV13 cessation) currently planned for after 2018. The logistics associated with\nglobally switching all use of trivalent OPV (tOPV) to bivalent OPV (bOPV) represent a significant undertaking, which\nmay cause some complications, including delays that lead to different timing of the switch across shared borders.\nMethods: Building on an integrated global model for long-term poliovirus risk management, we consider the expected\nvulnerability of different populations to transmission of OPV2-related polioviruses as a function of time following the\nswitch. We explore the relationship between the net reproduction number (Rn) of OPV2 at the time of the switch and the\ntime until OPV2-related viruses imported from countries still using OPV2 can establish transmission. We also analyze some\nspecific situations modeled after populations at high potential risk of circulating serotype 2 vaccine-derived poliovirus\n(cVDPV2) outbreaks in the event of a non-synchronous switch.\nResults: Well-implemented tOPV immunization activities prior to the tOPV to bOPV switch (i.e., tOPV intensification\nsufficient to prevent the creation of indigenous cVDPV2 outbreaks) lead to sufficient population immunity to transmission\nto cause die-out of any imported OPV2-related viruses for over 6 months after the switch in all populations in the global\nmodel. Higher Rn of OPV2 at the time of the switch reduces the time until imported OPV2-related viruses can establish\ntransmission and increases the time during which indigenous OPV2-related viruses circulate. Modeling specific connected\npopulations suggests a relatively low vulnerability to importations of OPV2-related viruses that could establish\ntransmission in the context of a non-synchronous switch from tOPV to bOPV, unless the gap between switch times\nbecomes very long (>6 months) or a high risk of indigenous cVDPV2s already exists in the importing and/or the\nexporting population.\nConclusions: Short national discrepancies in the timing of the tOPV to bOPV switch will likely not significantly increase\ncVDPV2 risks due to the insurance provided by tOPV intensification efforts, although the goal to coordinate national\nswitches within the globally agreed April 17-May 1, 2016 time window minimized the risks associated with cross-border\nimportations.
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