The risk of type 2 oral polio vaccine use in post-cessation outbreak response
Wild type 2 poliovirus was last observed in 1999. The Sabin-strain oral polio vaccine type 2 (OPV2) was critical to eradication, but it is known to revert to a neurovirulent phenotype, causing vaccine-associated paralytic poliomyelitis. OPV2 is also transmissible and can establish circulating lineages, called circulating vaccine-derived polioviruses (cVDPVs), which can also cause paralytic outbreaks. Thus, in April 2016, OPV2 was removed from immunization activities worldwide. Interrupting transmission of cVDPV2 lineages that survive cessation will require OPV2 in outbreak response, which risks seeding new cVDPVs. This potential cascade of outbreak responses seeding VDPVs, necessitating further outbreak responses, presents a critical risk to the OPV2 cessation effort.
The EMOD individual-based disease transmission model was used to investigate OPV2 use in outbreak response post-cessation in West African populations. A hypothetical outbreak response in northwest Nigeria is modeled, and a cVDPV2 lineage is considered established if the Sabin strain escapes the response region and continues circulating 9 months post-response. The probability of this event was investigated in a variety of possible scenarios.
Under a broad range of scenarios, the probability that widespread OPV2 use in outbreak response (~2 million doses) establishes new cVDPV2 lineages in this model may exceed 50% as soon as 18 months or as late as 4 years post-cessation.
Estimated probability with uncertainty of OPV2 survival for R 0f = 1.5 (gray), 2.0 (red), and 3.0 (cyan), vs. the time since cessation, at a fixed value of 0.001 for the mean per-person, per-day migration rate (or y = –3 on the log-space y-axes in Figs. 1, 3, 4, and 5). The red line and corresponding uncertainty band correspond to the estimated probability of OPV2 survival along a slice at y = –3 through the colored separatrix surface presented in Fig. 1; the cyan and gray areas represent the same quantity for simulated scenarios run with different values of the final VDPV infectivity. The other scenario parameters are set to g = 0.5, λ = 60 days, N IPV = 1, c = 1. The uncertainty bands represent uncertainty on the estimated probability of the OPV2 survival outcome at a given point in parameter space and do not incorporate uncertainty in the simulation input parameters themselves or other extrinsic sources of uncertainty