Background: Wild type 2 poliovirus was last observed in 1999. The Sabin-strain oral polio vaccine type 2 (OPV2)\nwas critical to eradication, but it is known to revert to a neurovirulent phenotype, causing vaccine-associated\nparalytic poliomyelitis. OPV2 is also transmissible and can establish circulating lineages, called circulating vaccinederived\npolioviruses (cVDPVs), which can also cause paralytic outbreaks. Thus, in April 2016, OPV2 was removed\nfrom immunization activities worldwide. Interrupting transmission of cVDPV2 lineages that survive cessation will\nrequire OPV2 in outbreak response, which risks seeding new cVDPVs. This potential cascade of outbreak responses\nseeding VDPVs, necessitating further outbreak responses, presents a critical risk to the OPV2 cessation effort.\nMethods: The EMOD individual-based disease transmission model was used to investigate OPV2 use in outbreak\nresponse post-cessation in West African populations. A hypothetical outbreak response in northwest Nigeria is modeled,\nand a cVDPV2 lineage is considered established if the Sabin strain escapes the response region and continues circulating\n9 months post-response. The probability of this event was investigated in a variety of possible scenarios.\nResults: Under a broad range of scenarios, the probability that widespread OPV2 use in outbreak response\n(~2 million doses) establishes new cVDPV2 lineages in this model may exceed 50% as soon as 18 months or\nas late as 4 years post-cessation.\nConclusions: The risk of a cycle in which outbreak responses seed new cVDPV2 lineages suggests that OPV2\nuse should be managed carefully as time from cessation increases. It is unclear whether this risk can be\nmitigated in the long term, as mucosal immunity against type 2 poliovirus declines globally. Therefore,\ncurrent programmatic strategies should aim to minimize the possibility that continued OPV2 use will be\nnecessary in future years: conducting rapid and aggressive outbreak responses where cVDPV2 lineages are\ndiscovered, maintaining high-quality surveillance in all high-risk settings, strengthening the use of the\ninactivated polio vaccine as a booster in the OPV2-exposed and in routine immunization, and gaining access\nto currently inaccessible areas of the world to conduct surveillance.
Loading....