The Last Mile in Polio Eradication: Program Challenges and Perseverance.

As the Global Polio Eradication Initiative (GPEI) strategizes towards the final steps of eradication, routine immunization schedules evolve, and high-quality vaccination campaigns and surveillance systems remain essential. New tools are consistently being developed, such as the novel oral poliovirus vaccine to combat outbreaks more sustainably, as well as non-infectiously manufactured vaccines such as virus-like particle vaccines to eliminate the risk of resurgence of polio on the eve of a polio-free world. As the GPEI inches towards eradication, re-strategizing in the face of evolving challenges and preparing for unknown risks in the post-certification era are critical.

Inactivated Poliovirus Vaccine: Recent Developments and the Tortuous Path to Global Acceptance.

Inactivated poliovirus vaccine (IPV), available since 1955, became the first vaccine to be used to protect against poliomyelitis. While the immunogenicity of IPV to prevent paralytic poliomyelitis continues to be irrefutable, its requirement for strong containment (due to large quantities of live virus used in the manufacturing process), perceived lack of ability to induce intestinal mucosal immunity, high cost and increased complexity to administer compared to oral polio vaccine (OPV), have limited its use in the global efforts to eradicate poliomyelitis. In order to harvest the full potential of IPV, a program of work has been carried out by the Global Polio Eradication Initiative (GPEI) over the past two decades that has focused on: (1) increasing the scientific knowledge base of IPV; (2) translating new insights and evidence into programmatic action; (3) expanding the IPV manufacturing infrastructure for global demand; and (4) continuing to pursue an ambitious research program to develop more immunogenic and safer-to-produce vaccines. While the knowledge base of IPV continues to expand, further research and product development are necessary to ensure that the program priorities are met (e.g., non-infectious production through virus-like particles, non-transmissible vaccine inducing humoral and intestinal mucosal immunity and new methods for house-to-house administration through micro-needle patches and jet injectors), the discussions have largely moved from whether to how to use this vaccine most effectively. In this review, we summarize recent developments on expanding the science base of IPV and provide insight into policy development and the expansion of IPV manufacturing and production, and finally we provide an update on the current priorities.

Enabling accelerated vaccine roll-out for Public Health Emergencies of International Concern (PHEICs): Novel Oral Polio Vaccine type 2 (nOPV2) experience.

To address the evolving risk of circulating vaccine-derived poliovirus type 2 (cVDPV2), Global Polio Eradication Initiative (GPEI) partners are working closely with countries to deploy an additional innovative tool for outbreak response - novel oral polio vaccine type 2 (nOPV2). The World Health Organization's (WHO) Prequalification program issued an Emergency Use Listing (EUL) recommendation for nOPV2 on 13 November 2020. The WHO's EUL procedure was created to assess and list unlicensed vaccines, therapeutics and diagnostics to enable their use in response to a Public Health Emergency of International Concern (PHEIC). nOPV2 was the first vaccine to receive an EUL, paving the way for other emergency vaccines. In this report, we summarise the pathway for nOPV2 roll-out under EUL.

Genetic Characterization of Novel Oral Polio Vaccine Type 2 Viruses During Initial Use Phase Under Emergency Use Listing - Worldwide, March-October 2021.

The emergence and international spread of neurovirulent circulating vaccine-derived polioviruses (cVDPVs) across multiple countries in Africa and Asia in recent years pose a major challenge to the goal of eradicating all forms of polioviruses. Approximately 90% of all cVDPV outbreaks are caused by the type 2 strain of the Sabin vaccine, an oral live, attenuated vaccine; cVDPV outbreaks typically occur in areas of persistently low immunization coverage (1). A novel type 2 oral poliovirus vaccine (nOPV2), produced by genetic modification of the type 2 Sabin vaccine virus genome (2), was developed and evaluated through phase I and phase II clinical trials during 2017-2019. nOPV2 was demonstrated to be safe and well-tolerated, have noninferior immunogenicity, and have superior genetic stability compared with Sabin monovalent type 2 (as measured by preservation of the primary attenuation site [domain V in the 5' noncoding region] and significantly lower neurovirulence of fecally shed vaccine virus in transgenic mice) (3-5). These findings indicate that nOPV2 could be an important tool in reducing the risk for generating vaccine-derived polioviruses (VDPVs) and the risk for vaccine-associated paralytic poliomyelitis cases. Based on the favorable preclinical and clinical data, and the public health emergency of international concern generated by ongoing endemic wild poliovirus transmission and cVDPV type 2 outbreaks, the World Health Organization authorized nOPV2 for use under the Emergency Use Listing (EUL) pathway in November 2020, allowing for its first use for outbreak response in March 2021 (6). As required by the EUL process, among other EUL obligations, an extensive plan was developed and deployed for obtaining and monitoring nOPV2 isolates detected during acute flaccid paralysis (AFP) surveillance, environmental surveillance, adverse events after immunization surveillance, and targeted surveillance for adverse events of special interest (i.e., prespecified events that have the potential to be causally associated with the vaccine product), during outbreak response, as well as through planned field studies. Under this monitoring framework, data generated from whole-genome sequencing of nOPV2 isolates, alongside other virologic data for isolates from AFP and environmental surveillance systems, are reviewed by the genetic characterization subgroup of an nOPV working group of the Global Polio Eradication Initiative. Global nOPV2 genomic surveillance during March-October 2021 confirmed genetic stability of the primary attenuating site. Sequence data generated through this unprecedented global effort confirm the genetic stability of nOPV2 relative to Sabin 2 and suggest that nOPV2 will be an important tool in the eradication of poliomyelitis. nOPV2 surveillance should continue for the duration of the EUL.

Synthesis, Properties, and Crystal Structures of Benzene-1,2-dithiolato Complexes of Thallium(I) and -(III).

The syntheses, molecular structures and properties of homoleptic 1,2-S(2)C(6)H(4) complexes of thallium(I) and thallium(III) with four-coordinated metal centers are described. Anaerobic treatment of TlCl, TlNO(3), or Tl(2)CO(3) with solutions of sodium methanolate and 1,2-(HS)(2)C(6)H(4) in methanol gave after metathesis with [NEt(4)]Br yellow solutions of [NEt(4)](2)[{Tl(1,2-(&mgr;-S)(2)C(6)H(4))}(2)] ([NEt(4)](2)1). Yellow single crystals were obtained from saturated acetone solutions at -10 degrees C and the crystal data for [NEt(4)](2)1 are monoclinic, P2(1)/c, with Z = 2, a = 7.440(1) Å, b = 16.373(3) Å, c = 13.201(2) Å, and beta = 97.08(1) degrees. Complex 1(2)(-)(), the first structurally characterized homoleptic ionic thiolate complex of thallium(I), contains rectangular bipyramidal [TlS(4)Tl] cages with the four sulfur atoms defining the equatorial plane and the two thallium atoms in axial positions. The S(2)C(6)H(4) fragments are almost coplanar with the S(4) plane. In the crystal lattice, nearly linear Tl.Tl chains align along the a-axis (offset ca. 3.0 degrees ) with the ligand planes parallel to the bc-plane. Within and between dimers short Tl.Tl distances are observed (Tl.Tl' within a dimeric unit, 3.5116(4) Å; Tl.Tl between dimeric units, 3.9371(3) Å) with the distance between dimeric units being the shortest contact between anions-Tl.S distances between dimeric units are longer than 5.8 Å. Aerobic treatment of TlCl, TlNO(3), or Tl(2)CO(3) with solutions of sodium methanolate and 1,2-(HS)(2)C(6)H(4) in methanol and metathesis with [NEt(4)]Br led to [NEt(4)][Tl(1,2-S(2)C(6)H(4))(2)] ([NEt(4)]2). Yellow single crystals were obtained from saturated acetone solutions at 0 degrees C and the crystal data for [NEt(4)]2 are orthorhombic, Pnn2, with Z = 2, a = 11.449(2) Å, b = 10.060(2) Å, c = 9.950(2) Å. Complex 2(-) is the first homoleptic four-coordinate thiolate of thallium(III) and contains the unusually short Tl-S distance of 2.469(4) Å. In solution, ion pairing results in chemical and magnetic inequivalence of the S(2)C(6)H(4) ligands. Although both preparations employ the reaction of thallium(I) salts with 1,2-(NaS)(2)C(6)H(4) in a 1:2 stoichiometry, complex 1(2)(-) is probably not an intermediate to the formation of 2(-). Exposing anaerobically prepared solutions of 1(2)(-) to air results in a series of color changes in the solution over a 20 min period; however, 2(-) could not be observed by NMR spectroscopy.