Implementation of coordinated global serotype 2 oral poliovirus vaccine cessation: risks of potential non-synchronous cessation

Abstract

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

Maintenance and Intensification of Bivalent Oral Poliovirus Vaccine Use Prior to its Coordinated Global Cessation

“…Doing so will minimize the risk of 1) continued circulation of indigenous serotype 2 cVDPV (cVDPV2) outbreaks that began before the switch [14,20,21], 2) continued transmission and evolution of OPV2-related viruses after the switch [14, 20], 3) cVDPV2 outbreaks due to importation of OPV2-related poliovirus into countries that already switched from countries yet to switch in the event of a non-synchronous switch [22], and 4) cVDPV2 outbreaks following inadvertent tOPV [23] or deliberate serotype 2 mOPV use after the switch [24]. …”

Cessation of Trivalent Oral Poliovirus Vaccine and Introduction of Inactivated Poliovirus Vaccine — Worldwide, 2016

“…Afghanistan, Nigeria, and Pakistan also conducted SIAs with IPV in selected regions before stopping tOPV use. In addition, the synchronized timing of the switch aimed to prevent exportations of type 2 polioviruses from areas continuing to use tOPV to neighboring areas that have ceased tOPV use (3,4). All 155 countries and territories that used OPV in 2015 reported that they had terminated use of tOPV by May 12, 2016 ( Figure 1).…”

“…Although the global cessation of tOPV use is essential for eliminating cVDPV2s, cessation of tOPV use carries some risks for facilitating the spread of undetected or newly emergent cVDPV2s among persons without immunity to type 2 poliovirus infections after the switch to bOPV (3)(4)(5). To stop the spread of existing cVDPV2s before the switch and to reduce risks for post-switch outbreaks (4), population immunity to type 2 poliovirus at the time of the switch was boosted through implementation of 116 supplemental immunization activities (SIAs ¶ ) with tOPV in 42 OPV-using countries during November 2015-April 2016.…”

Comprehensive screening for immunodeficiency-associated vaccine-derived poliovirus: an essential oral poliovirus vaccine cessation risk management strategy

“…First, the probability decreases that a contact between an iVDPV excretor and another individual in its subpopulation leads to an effective introduction that can start to transmit at the population level. Second, even effective introductions may die out before substantial circulation due to seasonality or other outbreak kinetics [14], or may stop more easily after an aggressive outbreak response. Consequently, introducing iVDPVs as partially reverted viruses yields large reductions in the probability of outbreaks and OPV restarts, expected outbreak response vaccine needs, and expected polio cases, which translates into higher incremental net benefits for both the base case without PAVDs and for the option with PAVDs.…”

“…Other limitations and uncertainties from the global model [23] and the underlying poliovirus transmission and OPV evolution model [33] carry over to this analysis. Limitations and uncertainties that may particularly affect PAVD benefits include the kinetics of outbreaks (both during the initial stages following a point introduction [14] and the frequency of long-range poliovirus exportations [23]), the impact of IPV-alone on population immunity to transmission in developing countries [10, 35], the relationship between population immunity to transmission and the probability that an iVDPV or other introduction establishes transmission, and efforts to manage cVDPV [10, 12–14] and laboratory containment risks [20, 36, 37]. The actual duration of mOPV use for outbreak response and the availability of any new poliovirus vaccines with lower risks than OPV may further affect the risk of uncontrolled outbreaks due to iVDPVs and the benefits of PAVDs [38].…”