McCarthy Kevin A, Chabot-Couture Guillaume, Famulare Michael, Lyons Hil M, Mercer Laina D
Institute for Disease Modeling, Bellevue, WA, USA.
BMC Med. 2017 Oct 4;15(1):175. doi: 10.1186/s12916-017-0937-y.
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.
The risk of a cycle in which outbreak responses seed new cVDPV2 lineages suggests that OPV2 use should be managed carefully as time from cessation increases. It is unclear whether this risk can be mitigated in the long term, as mucosal immunity against type 2 poliovirus declines globally. Therefore, current programmatic strategies should aim to minimize the possibility that continued OPV2 use will be necessary in future years: conducting rapid and aggressive outbreak responses where cVDPV2 lineages are discovered, maintaining high-quality surveillance in all high-risk settings, strengthening the use of the inactivated polio vaccine as a booster in the OPV2-exposed and in routine immunization, and gaining access to currently inaccessible areas of the world to conduct surveillance.
野生型2型脊髓灰质炎病毒最后一次被观察到是在1999年。2型脊髓灰质炎减毒活疫苗（OPV2）对根除脊髓灰质炎至关重要，但已知它会回复为神经毒性表型，导致疫苗相关麻痹性脊髓灰质炎。OPV2也具有传染性，能够形成循环谱系，即循环疫苗衍生脊髓灰质炎病毒（cVDPV），这也可引发麻痹性疫情。因此，2016年4月，OPV2被从全球免疫活动中移除。在疫情应对中，要阻断在停用后仍存活的cVDPV2谱系的传播，就需要使用OPV2，这有引发新的cVDPV的风险。这种可能引发VDPV的疫情应对级联反应，进而需要进一步的疫情应对措施，给OPV2停用工作带来了重大风险。
使用基于个体的EMOD疾病传播模型来研究在西非人群中停用OPV2后在疫情应对中使用该疫苗的情况。对尼日利亚西北部的一次假设性疫情应对进行建模，如果减毒活疫苗株逃出应对区域并在应对后9个月仍继续传播，则认为cVDPV2谱系已形成。在各种可能的情况下研究了这一事件的概率。
在广泛的情况下，在疫情应对中广泛使用OPV2（约200万剂）在该模型中引发新的cVDPV2谱系的概率，在停用后18个月或晚至4年时可能超过50%。
疫情应对引发新的cVDPV2谱系的循环风险表明，随着停用时间的增加，应谨慎管理OPV2的使用。由于全球范围内针对2型脊髓灰质炎病毒的黏膜免疫力下降，目前尚不清楚这种风险从长期来看是否能够减轻。因此，当前的规划策略应旨在尽量减少未来几年继续使用OPV2的可能性：在发现cVDPV2谱系的地方迅速采取积极的疫情应对措施，在所有高风险地区维持高质量监测，加强在接触过OPV2的人群和常规免疫中使用灭活脊髓灰质炎疫苗作为加强剂，并进入世界上目前难以进入的地区进行监测。
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