Worst-case scenarios: Modeling uncontrolled type 2 polio transmission

In May 2016, the Global Polio Eradication Initiative (GPEI) coordinated the cessation of all use of type 2 oral poliovirus vaccine (OPV2), except for emergency outbreak response. Since then, paralytic polio cases caused by type 2 vaccine-derived polioviruses now exceed 3,000 cases reported by 39 countries. In 2022 (as of April 25, 2023), 20 countries reported detection of cases and nine other countries reported environmental surveillance detection, but no reported cases. Recent development of a genetically modified novel type 2 OPV (nOPV2) may help curb the generation of neurovirulent vaccine-derived strains; its use since 2021 under Emergency Use Listing is limited to outbreak response activities. Prior modeling studies showed that the expected trajectory for global type 2 viruses does not appear headed toward eradication, even with the best possible properties of nOPV2 assuming current outbreak response performance. Continued persistence of type 2 poliovirus transmission exposes the world to the risks of potentially high-consequence events such as the importation of virus into high-transmission areas of India or Bangladesh. Building on prior polio endgame modeling and assuming current national and GPEI outbreak response performance, we show no probability of successfully eradicating type 2 polioviruses in the near term regardless of vaccine choice. We also demonstrate the possible worst-case scenarios could result in rapid expansion of paralytic cases and preclude the goal of permanently ending all cases of poliomyelitis in the foreseeable future. Avoiding such catastrophic scenarios will depend on the development of strategies that raise population immunity to type 2 polioviruses.

Coordinated global cessation of oral poliovirus vaccine use: Options and potential consequences

Due to the very low, but nonzero, paralysis risks associated with the use of oral poliovirus vaccine (OPV), eradicating poliomyelitis requires ending all OPV use globally. The Global Polio Eradication Initiative (GPEI) coordinated cessation of Sabin type 2 OPV (OPV2 cessation) in 2016, except for emergency outbreak response. However, as of early 2023, plans for cessation of bivalent OPV (bOPV, containing types 1 and 3 OPV) remain undefined, and OPV2 use for outbreak response continues due to ongoing transmission of type 2 polioviruses and reported type 2 cases. Recent development and use of a genetically stabilized novel type 2 OPV (nOPV2) leads to additional potential vaccine options and increasing complexity in strategies for the polio endgame. Prior applications of integrated global risk, economic, and poliovirus transmission modeling consistent with GPEI strategic plans that preceded OPV2 cessation explored OPV cessation dynamics and the evaluation of options to support globally coordinated risk management efforts. The 2022-2026 GPEI strategic plan highlighted the need for early bOPV cessation planning. We review the published modeling and explore bOPV cessation immunization options as of 2022, assuming that the GPEI partners will not support restart of the use of any OPV type in routine immunization after a globally coordinated cessation of such use. We model the potential consequences of globally coordinating bOPV cessation in 2027, as anticipated in the 2022-2026 GPEI strategic plan. We do not find any options for bOPV cessation likely to succeed without a strategy of bOPV intensification to increase population immunity prior to cessation.

Poliovirus type 1 systemic humoral and intestinal mucosal immunity induced by monovalent oral poliovirus vaccine, fractional inactivated poliovirus vaccine, and bivalent oral poliovirus vaccine: A randomized controlled trial

BACKGROUND

To inform response strategies, we examined type 1 humoral and intestinal immunity induced by 1) one fractional inactivated poliovirus vaccine (fIPV) dose given with monovalent oral poliovirus vaccine (mOPV1), and 2) mOPV1 versus bivalent OPV (bOPV).

METHODS

We conducted a randomized, controlled, open-label trial in Dhaka, Bangladesh. Healthy infants aged 5 weeks were block randomized to one of four arms: mOPV1 at age 6-10-14 weeks/fIPV at 6 weeks (A); mOPV1 at 6-10-14 weeks/fIPV at 10 weeks (B); mOPV1 at 6-10-14 weeks (C); and bOPV at 6-10-14 weeks (D). Immune response at 10 weeks and cumulative response at 14 weeks was assessed among the modified intention-to-treat population, defined as seroconversion from seronegative (<1:8 titers) to seropositive (≥1:8) or a four-fold titer rise among seropositive participants sustained to age 18 weeks. We examined virus shedding after two doses of mOPV1 with and without fIPV, and after the first mOPV1 or bOPV dose. The trial is registered at ClinicalTrials.gov (NCT03722004).

FINDINGS

During 18 December 2018 - 23 November 2019, 1,192 infants were enrolled (arms A:301; B:295; C:298; D:298). Immune responses at 14 weeks did not differ after two mOPV1 doses alone (94% [95% CI: 91-97%]) versus two mOPV1 doses with fIPV at 6 weeks (96% [93-98%]) or 10 weeks (96% [93-98%]). Participants who received mOPV1 and fIPV at 10 weeks had significantly lower shedding (p < 0·001) one- and two-weeks later compared with mOPV1 alone. Response to one mOPV1 dose was significantly higher than one bOPV dose (79% versus 67%; p < 0·001) and shedding two-weeks later was significantly higher after mOPV1 (76% versus 56%; p < 0·001) indicating improved vaccine replication. Ninety-nine adverse events were reported, 29 serious including two deaths; none were attributed to study vaccines.

INTERPRETATION

Given with the second mOPV1 dose, fIPV improved intestinal immunity but not humoral immunity. One mOPV1 dose induced higher humoral and intestinal immunity than bOPV.

FUNDING

U.S. Centers for Disease Control and Prevention.

Immunogenicity of novel oral poliovirus vaccine type 2 administered concomitantly with bivalent oral poliovirus vaccine: an open-label, non-inferiority, randomised, controlled trial

BACKGROUND

Novel oral poliovirus vaccine type 2 (nOPV2) was developed by modifying the Sabin strain to increase genetic stability and reduce risk of seeding new circulating vaccine-derived poliovirus type 2 outbreaks. Bivalent oral poliovirus vaccine (bOPV; containing Sabin types 1 and 3) is the vaccine of choice for type 1 and type 3 outbreak responses. We aimed to assess immunological interference between nOPV2 and bOPV when administered concomitantly.

METHODS

We conducted an open-label, non-inferiority, randomised, controlled trial at two clinical trial sites in Dhaka, Bangladesh. Healthy infants aged 6 weeks were randomly assigned (1:1:1) using block randomisation, stratified by site, to receive nOPV2 only, nOPV2 plus bOPV, or bOPV only, at the ages of 6 weeks, 10 weeks, and 14 weeks. Eligibility criteria included singleton and full term (≥37 weeks' gestation) birth and parents intending to remain in the study area for the duration of study follow-up activities. Poliovirus neutralising antibody titres were measured at the ages of 6 weeks, 10 weeks, 14 weeks, and 18 weeks. The primary outcome was cumulative immune response for all three poliovirus types at the age of 14 weeks (after two doses) and was assessed in the modified intention-to-treat population, which was restricted to participants with adequate blood specimens from all study visits. Safety was assessed in all participants who received at least one dose of study product. A non-inferiority margin of 10% was used to compare single and concomitant administration. This trial is registered with ClinicalTrials.gov, NCT04579510.

FINDINGS

Between Feb 8 and Sept 26, 2021, 736 participants (244 in the nOPV2 only group, 246 in the nOPV2 plus bOPV group, and 246 in the bOPV only group) were enrolled and included in the modified intention-to-treat analysis. After two doses, 209 (86%; 95% CI 81-90) participants in the nOPV2 only group and 159 (65%; 58-70) participants in the nOPV2 plus bOPV group had a type 2 poliovirus immune response; 227 (92%; 88-95) participants in the nOPV2 plus bOPV group and 229 (93%; 89-96) participants in the bOPV only group had a type 1 response; and 216 (88%; 83-91) participants in the nOPV2 plus bOPV group and 212 (86%; 81-90) participants in the bOPV only group had a type 3 response. Co-administration was non-inferior to single administration for types 1 and 3, but not for type 2. There were 15 serious adverse events (including three deaths, one in each group, all attributable to sudden infant death syndrome); none were attributed to vaccination.

INTERPRETATION

Co-administration of nOPV2 and bOPV interfered with immunogenicity for poliovirus type 2, but not for types 1 and 3. The blunted nOPV2 immunogenicity we observed would be a major drawback of using co-administration as a vaccination strategy.

FUNDING

The US Centers for Disease Control and Prevention.

Progress Toward Poliomyelitis Eradication - Worldwide, January 2021-March 2023

Since the World Health Assembly established the Global Polio Eradication Initiative (GPEI) in 1988, two of the three wild poliovirus (WPV) serotypes (types 2 and 3) have been eradicated, and global WPV cases have decreased by more than 99.9%. Afghanistan and Pakistan remain the only countries where indigenous WPV type 1 (WPV1) transmission has not been interrupted. This report summarizes progress toward global polio eradication during January 1, 2021-March 31, 2023, and updates previous reports (1,2). In 2022, Afghanistan and Pakistan reported 22 WPV1 cases, compared with five in 2021; as of May 5, 2023, a single WPV1 case was reported in Pakistan in 2023. A WPV1 case was reported on the African continent for the first time since 2016, when officials in Malawi confirmed a WPV1 case in a child with paralysis onset in November 2021; neighboring Mozambique subsequently reported eight genetically linked cases. Outbreaks of polio caused by circulating vaccine-derived polioviruses (cVDPVs) can occur when oral poliovirus vaccine (OPV) strains circulate for a prolonged time in underimmunized populations, allowing reversion to neurovirulence (3). A total of 859 cVDPV cases occurred during 2022, an increase of 23% from 698 cases in 2021. cVDPVs were detected in areas where poliovirus transmission had long been eliminated (including in Canada, Israel, the United Kingdom, and the United States). In addition, cocirculation of multiple poliovirus types occurred in multiple countries globally (including Democratic Republic of the Congo [DRC], Israel, Malawi, Mozambique, Republic of the Congo, and Yemen). The 2022-2026 GPEI strategic plan targeted the goal of detecting the last cases of WPV1 and cVDPV in 2023 (4). The current global epidemiology of poliovirus transmission makes the likelihood of meeting this target date unlikely. The detections of poliovirus (WPV1 and cVDPVs) in areas where it had been previously eliminated underscore the threat of continued poliovirus spread to any area where there is insufficient vaccination to poliovirus (3). Mass vaccination and surveillance should be further enhanced in areas of transmission to interrupt poliovirus transmission and to end the global threat of paralytic polio in children.

Update on Vaccine-Derived Poliovirus Outbreaks - Worldwide, January 2021-December 2022

Circulating vaccine-derived poliovirus (cVDPV) outbreaks* can occur when oral poliovirus vaccine (OPV, containing one or more Sabin-strain serotypes 1, 2, and 3) strains undergo prolonged circulation in under-vaccinated populations, resulting in genetically reverted neurovirulent virus (1,2). Following declaration of the eradication of wild poliovirus type 2 in 2015 and the global synchronized switch from trivalent OPV (tOPV, containing Sabin-strain types 1, 2, and 3) to bivalent OPV (bOPV, containing types 1 and 3 only) for routine immunization activities^† in April 2016 (3), cVDPV type 2 (cVDPV2) outbreaks have been reported worldwide (4). During 2016-2020, immunization responses to cVDPV2 outbreaks required use of Sabin-strain monovalent OPV2, but new VDPV2 emergences could occur if campaigns did not reach a sufficiently high proportion of children. Novel oral poliovirus vaccine type 2 (nOPV2), a more genetically stable vaccine than Sabin OPV2, was developed to address the risk for reversion to neurovirulence and became available in 2021. Because of the predominant use of nOPV2 during the reporting period, supply replenishment has frequently been insufficient for prompt response campaigns (5). This report describes global cVDPV outbreaks during January 2021-December 2022 (as of February 14, 2023) and updates previous reports (4). During 2021-2022, there were 88 active cVDPV outbreaks, including 76 (86%) caused by cVDPV2. cVDPV outbreaks affected 46 countries, 17 (37%) of which reported their first post-switch cVDPV2 outbreak. The total number of paralytic cVDPV cases during 2020-2022 decreased by 36%, from 1,117 to 715; however, the proportion of all cVDPV cases that were caused by cVDPV type 1 (cVDPV1) increased from 3% in 2020 to 18% in 2022, including the occurrence of cocirculating cVDPV1 and cVDPV2 outbreaks in two countries. The increased proportion of cVDPV1 cases follows a substantial decrease in global routine immunization coverage and suspension of preventive immunization campaigns during the COVID-19 pandemic (2020-2022) (6); outbreak responses in some countries were also suboptimal. Improving routine immunization coverage, strengthening poliovirus surveillance, and conducting timely and high-quality supplementary immunization activities (SIAs) in response to cVDPV outbreaks are needed to interrupt cVDPV transmission and reach the goal of no cVDPV isolations in 2024.

Serotype 2 oral poliovirus vaccine (OPV2) choices and the consequences of delaying outbreak response

The Global Polio Eradication Initiative (GPEI) faces substantial challenges with managing outbreaks of serotype 2 circulating vaccine-derived polioviruses (cVDPV2s) in 2021. A full five years after the globally coordinated removal of serotype 2 oral poliovirus vaccine (OPV2) from trivalent oral poliovirus vaccine (tOPV) for use in national immunization programs, cVDPV2s did not die out. Since OPV2 cessation, responses to outbreaks caused by cVDPV2s mainly used serotype 2 monovalent OPV (mOPV2) from a stockpile. A novel vaccine developed from a genetically stabilized OPV2 strain (nOPV2) promises to potentially facilitate outbreak response with lower prospective risks, although its availability and properties in the field remain uncertain. Using an established global poliovirus transmission model and building on a related analysis that characterized the impacts of disruptions in GPEI activities caused by the COVID-19 pandemic, we explore the implications of trade-offs associated with delaying outbreak response to avoid using mOPV2 by waiting for nOPV2 availability (or equivalently, delayed responses waiting for national validation of meeting the criteria for nOPV2 initial use). Consistent with prior modeling, responding as quickly as possible with available mOPV2 promises to reduce the expected burden of disease in the outbreak population and to reduce the chances for the outbreak virus to spread to other areas. Delaying cVDPV2 outbreak response (e.g., modeled as no response January-June 2021) to wait for nOPV2 can considerably increase the total expected cases (e.g., by as many as 1,300 cVDPV2 cases in the African region during 2021-2023) and increases the likelihood of triggering the need to restart widescale preventive use of an OPV2-containing vaccine in national immunization programs that use OPV. Countries should respond to any cVDPV2 outbreaks quickly with rounds that achieve high coverage using any available OPV2, and plan to use nOPV2, if needed, once it becomes widely available based on evidence that it is as effective but safer in populations than mOPV2.

Outbreak response strategies with type 2-containing oral poliovirus vaccines

Despite exhaustive and fully-financed plans to manage the risks of globally coordinated cessation of oral poliovirus vaccine (OPV) containing type 2 (OPV2) prior to 2016, as of 2022, extensive, continued transmission of circulating vaccine-derived polioviruses (cVDPVs) type 2 (cVDPV2) remains. Notably, cumulative cases caused by cVDPV2 since 2016 now exceed 2,500. Earlier analyses explored the implications of using different vaccine formulations to respond to cVDPV2 outbreaks and demonstrated how different properties of novel OPV2 (nOPV2) might affect its performance compared to Sabin monovalent OPV2 (mOPV2). These prior analyses used fixed assumptions for how outbreak response would occur, but outbreak response implementation can change. We update an existing global poliovirus transmission model to explore different options for responding with different vaccines and assumptions about scope, delays, immunization intensity, target age groups, and number of rounds. Our findings suggest that in order to successfully stop all cVDPV2 transmission globally, countries and the Global Polio Eradication Initiative need to address the deficiencies in emergency outbreak response policy and implementation. The polio program must urgently act to substantially reduce response time, target larger populations - particularly in high transmission areas - and achieve high coverage with improved access to under-vaccinated subpopulations. Given the limited supplies of nOPV2 at the present, using mOPV2 intensively immediately, followed by nOPV2 intensively if needed and when sufficient quantities become available, substantially increases the probability of ending cVDPV2 transmission globally.

Updated Characterization of Outbreak Response Strategies for 2019-2029: Impacts of Using a Novel Type 2 Oral Poliovirus Vaccine Strain

Delays in achieving the global eradication of wild poliovirus transmission continue to postpone subsequent cessation of all oral poliovirus vaccine (OPV) use. Countries must stop OPV use to end all cases of poliomyelitis, including vaccine-associated paralytic polio (VAPP) and cases caused by vaccine-derived polioviruses (VDPVs). The Global Polio Eradication Initiative (GPEI) coordinated global cessation of all type 2 OPV (OPV2) use in routine immunization in 2016 but did not successfully end the transmission of type 2 VDPVs (VDPV2s), and consequently continues to use type 2 OPV (OPV2) for outbreak response activities. Using an updated global poliovirus transmission and OPV evolution model, we characterize outbreak response options for 2019-2029 related to responding to VDPV2 outbreaks with a genetically stabilized novel OPV (nOPV2) strain or with the currently licensed monovalent OPV2 (mOPV2). Given uncertainties about the properties of nOPV2, we model different assumptions that appear consistent with the evidence on nOPV2 to date. Using nOPV2 to respond to detected cases may reduce the expected VDPV and VAPP cases and the risk of needing to restart OPV2 use in routine immunization compared to mOPV2 use for outbreak response. The actual properties, availability, and use of nOPV2 will determine its effects on type 2 poliovirus transmission in populations. Even with optimal nOPV2 performance, countries and the GPEI would still likely need to restart OPV2 use in routine immunization in OPV-using countries if operational improvements in outbreak response to stop the transmission of cVDPV2s are not implemented effectively.

Updated Characterization of Post-OPV Cessation Risks: Lessons from 2019 Serotype 2 Outbreaks and Implications for the Probability of OPV Restart

After the globally coordinated cessation of any serotype of oral poliovirus vaccine (OPV), some risks remain from undetected, existing homotypic OPV-related transmission and/or restarting transmission due to several possible reintroduction risks. The Global Polio Eradication Initiative (GPEI) coordinated global cessation of serotype 2-containing OPV (OPV2) in 2016. Following OPV2 cessation, the GPEI and countries implemented activities to withdraw all the remaining trivalent OPV, which contains all three poliovirus serotypes (i.e., 1, 2, and 3), from the supply chain and replace it with bivalent OPV (containing only serotypes 1 and 3). However, as of early 2020, monovalent OPV2 use for outbreak response continues in many countries. In addition, outbreaks observed in 2019 demonstrated evidence of different types of risks than previously modeled. We briefly review the 2019 epidemiological experience with serotype 2 live poliovirus outbreaks and propose a new risk for unexpected OPV introduction for inclusion in global modeling of OPV cessation. Using an updated model of global poliovirus transmission and OPV evolution with and without consideration of this new risk, we explore the implications of the current global situation with respect to the likely need to restart preventive use of OPV2 in OPV-using countries. Simulation results without this new risk suggest OPV2 restart will likely need to occur (81% of 100 iterations) to manage the polio endgame based on the GPEI performance to date with existing vaccine tools, and with the new risk of unexpected OPV introduction the expected OPV2 restart probability increases to 89%. Contingency planning requires new OPV2 bulk production, including genetically stabilized OPV2 strains.

Global Transmission of Live Polioviruses: Updated Dynamic Modeling of the Polio Endgame

Nearly 20 years after the year 2000 target for global wild poliovirus (WPV) eradication, live polioviruses continue to circulate with all three serotypes posing challenges for the polio endgame. We updated a global differential equation-based poliovirus transmission and stochastic risk model to include programmatic and epidemiological experience through January 2020. We used the model to explore the likely dynamics of poliovirus transmission for 2019-2023, which coincides with a new Global Polio Eradication Initiative Strategic Plan. The model stratifies the global population into 72 blocks, each containing 10 subpopulations of approximately 10.7 million people. Exported viruses go into subpopulations within the same block and within groups of blocks that represent large preferentially mixing geographical areas (e.g., continents). We assign representative World Bank income levels to the blocks along with polio immunization and transmission assumptions, which capture some of the heterogeneity across countries while still focusing on global poliovirus transmission dynamics. We also updated estimates of reintroduction risks using available evidence. The updated model characterizes transmission dynamics and resulting polio cases consistent with the evidence through 2019. Based on recent epidemiological experience and prospective immunization assumptions for the 2019-2023 Strategic Plan, the updated model does not show successful eradication of serotype 1 WPV by 2023 or successful cessation of oral poliovirus vaccine serotype 2-related viruses.

Surveillance to Track Progress Toward Polio Eradication - Worldwide, 2018-2019

Since the Global Polio Eradication Initiative (GPEI) was launched in 1988, the number of polio cases worldwide has declined approximately 99.99%; only two countries (Afghanistan and Pakistan) have never interrupted wild poliovirus (WPV) transmission (1). The primary means of detecting poliovirus circulation is through surveillance for acute flaccid paralysis (AFP) among children aged <15 years with testing of stool specimens for WPV and vaccine-derived polioviruses (VDPVs) (genetically reverted strains of the vaccine virus that regain neurovirulence) in World Health Organization (WHO)-accredited laboratories (2,3). In many locations, AFP surveillance is supplemented by environmental surveillance, the regular collection and testing of sewage to provide awareness of the extent and duration of poliovirus circulation (3). This report presents 2018-2019 poliovirus surveillance data, focusing on 40 priority countries* with WPV or VDPV outbreaks or at high risk for importation because of their proximity to a country with an outbreak. The number of priority countries rose from 31 in 2018 to 40 in 2019 because of a substantial increase in the number of VDPV outbreaks^† (2,4). In areas with low poliovirus immunity, VDPVs can circulate in the community and cause outbreaks of paralysis; these are known as circulating vaccine derived polioviruses (cVDPVs) (4). In 2019, only 25 (63%) of the 40 designated priority countries met AFP surveillance indicators nationally; subnational surveillance performance varied widely and indicated focal weaknesses. High quality, sensitive surveillance is important to ensure timely detection and response to cVDPV and WPV transmission.

Update on Vaccine-Derived Poliovirus Outbreaks - Democratic Republic of the Congo and Horn of Africa, 2017-2018

Widespread use of live attenuated (Sabin) oral poliovirus vaccine (OPV) has resulted in marked progress toward global poliomyelitis eradication (1). However, in underimmunized populations, extensive person-to-person transmission of Sabin poliovirus can result in genetic reversion to neurovirulence and paralytic vaccine-derived poliovirus (VDPV) disease (1). This report updates (as of February 26, 2019) previous reports on circulating VDPV type 2 (cVDPV2) outbreaks during 2017-2018 in the Democratic Republic of the Congo (DRC) and in Somalia, which experienced a concurrent cVDPV type 3 (cVDPV3) outbreak* (2,3). In DRC, 42 cases have been reported in four cVDPV2 outbreaks; paralysis onset in the most recent case was October 7, 2018 (2). Challenges to interrupting transmission have included delays in outbreak-response supplementary immunization activities (SIAs) and difficulty reaching children in all areas. In Somalia, cVDPV2 and cVDPV3 were detected in sewage before the detection of paralytic cases (3). Twelve type 2 and type 3 cVDPV cases have been confirmed; the most recent paralysis onset dates were September 2 (cVDPV2) and September 7, 2018 (cVDPV3). The primary challenge to interrupting transmission is the residence of >300,000 children in areas that are inaccessible for vaccination activities. For both countries, longer periods of surveillance are needed before interruption of cVDPV transmission can be inferred.

Update on Vaccine-Derived Polioviruses - Worldwide, January 2016-June 2017

In 1988, the World Health Assembly launched the Global Polio Eradication Initiative (GPEI) (1). Among the three wild poliovirus (WPV) serotypes, only type 1 (WPV1) has been detected since 2012. Since 2014, detection of WPV1 has been limited to three countries, with 37 cases in 2016 and 11 cases in 2017 as of September 27. The >99.99% decline worldwide in polio cases since the launch of the GPEI is attributable to the extensive use of the live, attenuated oral poliovirus vaccine (OPV) in mass vaccination campaigns and comprehensive national routine immunization programs. Despite its well-established safety record, OPV use can be associated with rare emergence of genetically divergent vaccine-derived polioviruses (VDPVs) whose genetic drift from the parental OPV strains indicates prolonged replication or circulation (2). VDPVs can also emerge among persons with primary immunodeficiencies (PIDs). Immunodeficiency-associated VDPVs (iVDPVs) can replicate for years in some persons with PIDs. In addition, circulating vaccine-derived polioviruses (cVDPVs) can emerge very rarely among immunologically normal vaccine recipients and their contacts in areas with inadequate OPV coverage and can cause outbreaks of paralytic polio. This report updates previous summaries regarding VDPVs (3). During January 2016-June 2017, new cVDPV outbreaks were identified, including two in the Democratic Republic of the Congo (DRC) (eight cases), and another in Syria (35 cases), whereas the circulation of cVDPV type 2 (cVDPV2) in Nigeria resulted in cVDPV2 detection linked to a previous emergence. The last confirmed case from the 2015-2016 cVDPV type 1 (cVDPV1) outbreak in Laos occurred in January 2016. Fourteen newly identified persons in 10 countries were found to excrete iVDPVs, and three previously reported patients in the United Kingdom and Iran (3) were still excreting type 2 iVDPV (iVDPV2) during the reporting period. Ambiguous VDPVs (aVDPVs), isolates that cannot be classified definitively, were found among immunocompetent persons and environmental samples in 10 countries. Cessation of all OPV use after certification of polio eradication will eliminate the risk for new VDPV infections.

Using Acute Flaccid Paralysis Surveillance as a Platform for Vaccine-Preventable Disease Surveillance

Surveillance for acute flaccid paralysis (AFP) is a fundamental cornerstone of the global polio eradication initiative (GPEI). Active surveillance (with visits to health facilities) is a critical strategy of AFP surveillance systems for highly sensitive and timely detection of cases. Because of the extensive resources devoted to AFP surveillance, multiple opportunities exist for additional diseases to be added using GPEI assets, particularly because there is generally 1 district officer responsible for all disease surveillance. For this reason, integrated surveillance has become a standard practice in many countries, ranging from adding surveillance for measles and rubella to integrated disease surveillance for outbreak-prone diseases (integrated disease surveillance and response). This report outlines the current level of disease surveillance integration in 3 countries (Nepal, India, and Nigeria) and proposes that resources continue for long-term maintenance in resource-poor countries of AFP surveillance as a platform for surveillance of vaccine-preventable diseases and other outbreak-prone diseases.

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

OBJECTIVE

To examine the impact of different bivalent oral poliovirus vaccine (bOPV) supplemental immunization activity (SIA) strategies on population immunity to serotype 1 and 3 poliovirus transmission and circulating vaccine-derived poliovirus (cVDPV) risks before and after globally-coordinated cessation of serotype 1 and 3 oral poliovirus vaccine (OPV13 cessation).

METHODS

We adapt mathematical models that previously informed vaccine choices ahead of the trivalent oral poliovirus vaccine to bOPV switch to estimate the population immunity to serotype 1 and 3 poliovirus transmission needed at the time of OPV13 cessation to prevent subsequent cVDPV outbreaks. We then examine the impact of different frequencies of SIAs using bOPV in high risk populations on population immunity to serotype 1 and 3 transmission, on the risk of serotype 1 and 3 cVDPV outbreaks, and on the vulnerability to any imported bOPV-related polioviruses.

RESULTS

Maintaining high population immunity to serotype 1 and 3 transmission using bOPV SIAs significantly reduces 1) the risk of outbreaks due to imported serotype 1 and 3 viruses, 2) the emergence of indigenous cVDPVs before or after OPV13 cessation, and 3) the vulnerability to bOPV-related polioviruses in the event of non-synchronous OPV13 cessation or inadvertent bOPV use after OPV13 cessation.

CONCLUSION

Although some reduction in global SIA frequency can safely occur, countries with suboptimal routine immunization coverage should each continue to conduct at least one annual SIA with bOPV, preferably more, until global OPV13 cessation. Preventing cVDPV risks after OPV13 cessation requires investments in bOPV SIAs now through the time of OPV13 cessation.

Characterization of outbreak response strategies and potential vaccine stockpile needs for the polio endgame

BACKGROUND

Following successful eradication of wild polioviruses and planned globally-coordinated cessation of oral poliovirus vaccine (OPV), national and global health leaders may need to respond to outbreaks from reintroduced live polioviruses, particularly vaccine-derived polioviruses (VDPVs). Preparing outbreak response plans and assessing potential vaccine needs from an emergency stockpile require consideration of the different national risks and conditions as they change with time after OPV cessation.

METHODS

We used an integrated global model to consider several key issues related to managing poliovirus risks and outbreak response, including the time interval during which monovalent OPV (mOPV) can be safely used following homotypic OPV cessation; the timing, quality, and quantity of rounds required to stop transmission; vaccine stockpile needs; and the impacts of vaccine choices and surveillance quality. We compare the base case scenario that assumes aggressive outbreak response and sufficient mOPV available from the stockpile for all outbreaks that occur in the model, with various scenarios that change the outbreak response strategies.

RESULTS

Outbreak response after OPV cessation will require careful management, with some circumstances expected to require more and/or higher quality rounds to stop transmission than others. For outbreaks involving serotype 2, using trivalent OPV instead of mOPV2 following cessation of OPV serotype 2 but before cessation of OPV serotypes 1 and 3 would represent a good option if logistically feasible. Using mOPV for outbreak response can start new outbreaks if exported outside the outbreak population into populations with decreasing population immunity to transmission after OPV cessation, but failure to contain outbreaks resulting in exportation of the outbreak poliovirus may represent a greater risk. The possibility of mOPV use generating new long-term poliovirus excretors represents a real concern. Using the base case outbreak response assumptions, we expect over 25% probability of a shortage of stockpiled filled mOPV vaccine, which could jeopardize the achievement of global polio eradication. For the long term, responding to any poliovirus reintroductions may require a global IPV stockpile. Despite the risks, our model suggests that good risk management and response strategies can successfully control most potential outbreaks after OPV cessation.

CONCLUSIONS

Health leaders should carefully consider the numerous outbreak response choices that affect the probability of successfully managing poliovirus risks after OPV cessation.

Characterization of outbreak response strategies and potential vaccine stockpile needs for the polio endgame

BACKGROUND

Following successful eradication of wild polioviruses and planned globally-coordinated cessation of oral poliovirus vaccine (OPV), national and global health leaders may need to respond to outbreaks from reintroduced live polioviruses, particularly vaccine-derived polioviruses (VDPVs). Preparing outbreak response plans and assessing potential vaccine needs from an emergency stockpile require consideration of the different national risks and conditions as they change with time after OPV cessation.

METHODS

We used an integrated global model to consider several key issues related to managing poliovirus risks and outbreak response, including the time interval during which monovalent OPV (mOPV) can be safely used following homotypic OPV cessation; the timing, quality, and quantity of rounds required to stop transmission; vaccine stockpile needs; and the impacts of vaccine choices and surveillance quality. We compare the base case scenario that assumes aggressive outbreak response and sufficient mOPV available from the stockpile for all outbreaks that occur in the model, with various scenarios that change the outbreak response strategies.

RESULTS

Outbreak response after OPV cessation will require careful management, with some circumstances expected to require more and/or higher quality rounds to stop transmission than others. For outbreaks involving serotype 2, using trivalent OPV instead of mOPV2 following cessation of OPV serotype 2 but before cessation of OPV serotypes 1 and 3 would represent a good option if logistically feasible. Using mOPV for outbreak response can start new outbreaks if exported outside the outbreak population into populations with decreasing population immunity to transmission after OPV cessation, but failure to contain outbreaks resulting in exportation of the outbreak poliovirus may represent a greater risk. The possibility of mOPV use generating new long-term poliovirus excretors represents a real concern. Using the base case outbreak response assumptions, we expect over 25% probability of a shortage of stockpiled filled mOPV vaccine, which could jeopardize the achievement of global polio eradication. For the long term, responding to any poliovirus reintroductions may require a global IPV stockpile. Despite the risks, our model suggests that good risk management and response strategies can successfully control most potential outbreaks after OPV cessation.

CONCLUSIONS

Health leaders should carefully consider the numerous outbreak response choices that affect the probability of successfully managing poliovirus risks after OPV cessation.

An economic analysis of poliovirus risk management policy options for 2013-2052

Background

The Global Polio Eradication Initiative plans for coordinated cessation of oral poliovirus vaccine (OPV) after interrupting all wild poliovirus (WPV) transmission, but many questions remain related to long-term poliovirus risk management policies.

Methods

We used an integrated dynamic poliovirus transmission and stochastic risk model to simulate possible futures and estimate the health and economic outcomes of maintaining the 2013 status quo of continued OPV use in most developing countries compared with OPV cessation policies with various assumptions about global inactivated poliovirus vaccine (IPV) adoption.

Results

Continued OPV use after global WPV eradication leads to continued high costs and/or high cases. Global OPV cessation comes with a high probability of at least one outbreak, which aggressive outbreak response can successfully control in most instances. A low but non-zero probability exists of uncontrolled outbreaks following a poliovirus reintroduction long after OPV cessation in a population in which IPV-alone cannot prevent poliovirus transmission. We estimate global incremental net benefits during 2013–2052 of approximately $16 billion (US$2013) for OPV cessation with at least one IPV routine immunization dose in all countries until 2024 compared to continued OPV use, although significant uncertainty remains associated with the frequency of exportations between populations and the implementation of long term risk management policies.

Conclusions

Global OPV cessation offers the possibility of large future health and economic benefits compared to continued OPV use. Long-term poliovirus risk management interventions matter (e.g., IPV use duration, outbreak response, containment, continued surveillance, stockpile size and contents, vaccine production site requirements, potential antiviral drugs, and potential safer vaccines) and require careful consideration. Risk management activities can help to ensure a low risk of uncontrolled outbreaks and preserve or further increase the positive net benefits of OPV cessation. Important uncertainties will require more research, including characterizing immunodeficient long-term poliovirus excretor risks, containment risks, and the kinetics of outbreaks and response in an unprecedented world without widespread live poliovirus exposure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-1112-8) contains supplementary material, which is available to authorized users.

An economic analysis of poliovirus risk management policy options for 2013-2052

BACKGROUND

The Global Polio Eradication Initiative plans for coordinated cessation of oral poliovirus vaccine (OPV) after interrupting all wild poliovirus (WPV) transmission, but many questions remain related to long-term poliovirus risk management policies.

METHODS

We used an integrated dynamic poliovirus transmission and stochastic risk model to simulate possible futures and estimate the health and economic outcomes of maintaining the 2013 status quo of continued OPV use in most developing countries compared with OPV cessation policies with various assumptions about global inactivated poliovirus vaccine (IPV) adoption.

RESULTS

Continued OPV use after global WPV eradication leads to continued high costs and/or high cases. Global OPV cessation comes with a high probability of at least one outbreak, which aggressive outbreak response can successfully control in most instances. A low but non-zero probability exists of uncontrolled outbreaks following a poliovirus reintroduction long after OPV cessation in a population in which IPV-alone cannot prevent poliovirus transmission. We estimate global incremental net benefits during 2013-2052 of approximately $16 billion (US$2013) for OPV cessation with at least one IPV routine immunization dose in all countries until 2024 compared to continued OPV use, although significant uncertainty remains associated with the frequency of exportations between populations and the implementation of long term risk management policies.

CONCLUSIONS

Global OPV cessation offers the possibility of large future health and economic benefits compared to continued OPV use. Long-term poliovirus risk management interventions matter (e.g., IPV use duration, outbreak response, containment, continued surveillance, stockpile size and contents, vaccine production site requirements, potential antiviral drugs, and potential safer vaccines) and require careful consideration. Risk management activities can help to ensure a low risk of uncontrolled outbreaks and preserve or further increase the positive net benefits of OPV cessation. Important uncertainties will require more research, including characterizing immunodeficient long-term poliovirus excretor risks, containment risks, and the kinetics of outbreaks and response in an unprecedented world without widespread live poliovirus exposure.

Immunogenicity of three doses of bivalent, trivalent, or type 1 monovalent oral poliovirus vaccines with a 2 week interval between doses in Bangladesh: an open-label, non-inferiority, randomised, controlled trial

BACKGROUND

The provision of several doses of monovalent type 1 oral poliovirus vaccine (mOPV1) and bivalent OPV1 and 3 (bOPV) vaccines through campaigns is essential to stop the circulation of remaining wild polioviruses. Our study aimed to assess the shortening of intervals between campaigns with bOPV and mOPV1 and to assess the immunogenicity of bOPV in routine immunisation schedules.

METHODS

We did an open-label, non-inferiority, five-arm, randomised controlled trial in Bangladesh. We recruited healthy infants aged 6 weeks at 42 immunisation clinics and randomly assigned them (with blocks of 15, three per group) to receive a short three-dose schedule of bOPV (bOPV short) or mOPV1 (mOPV1 short) with the first dose given at age 6 weeks, the second at age 8 weeks, and the third at age 10 weeks; or to a standard three-dose schedule of bOPV (bOPV standard) or mOPV1 (mOPV1 standard) or trivalent OPV (tOPV standard) with the first dose given at age 6 weeks, the second at 10 weeks, and the third at age 14 weeks. The primary outcome was the proportion of infants with antibody seroconversion for type 1, type 2, and type 3 polioviruses. The primary, modified intention-to-treat analysis included all patients who had testable serum samples before and after receiving at least one OPV dose. We used a 10% margin to establish non-inferiority for bOPV groups versus mOPV1 groups in seroconversion for type 1 poliovirus, and for bOPV1 short versus bOPV1 standard for types 1 and 3. This trial is registered at ClinicalTrials.gov, number NCT01633216, and is closed to new participants.

FINDINGS

Between May 13, 2012, and Jan 21, 2013, we randomly assigned 1000 infants to our study groups. 927 completed all study visits and were included in the primary analysis. Seroconversion for type-1 poliovirus was recorded in 183 (98%, 95% CI 95-100) of 186 infants given bOPV short, 179 (97%, 94-99) of 184 given bOPV standard, 180 (96%, 92-98) of 188 given mOPV short, 178 (99%, 97-100) of 179 given mOPV1 standard, and 175 (92%, 87-96) of 190 given tOPV standard. Seroconversion for type 2 was noted in 16 infants (9%, 5-14) on bOPV short, 29 (16%, 11-22) on bOPV standard, 19 (10%, 7-15) on mOPV short, 33 (18%, 13-25) on mOPV1 standard, and 182 (96%, 92-98) on tOPV standard. Seroconversion for type 3 was noted in 175 infants (94%, 90-97) on bOPV short, 176 (96%, 92-98) on bOPV standard, 18 (10%, 6-15) on mOPV short, 25 (14%, 10-20) on mOPV1 standard, and 167 (88%, 83-92) on tOPV standard. The short schedules for mOPV1 and bOPV elicited a non-inferior antibody response compared with the bOPV standard schedule. 104 adverse events were reported in 100 infants during follow up. 36 of these events needed admission to hospital (32 were pneumonia, two were vomiting or feeding disorders, one was septicaemia, and one was diarrhoea with severe malnutrition). One of the infants admitted to hospital for pneumonia died 5 days after admission. No adverse event was attributed to the vaccines.

INTERPRETATION

Our trial showed that three doses of mOPV1 or bOPV with a short schedule of 2 week intervals between doses induces an immune response similar to that obtained with the standard schedule of giving doses at 4 week intervals. These findings support the use of these vaccines in campaigns done at short intervals to rapidly increase population immunity against polioviruses to control outbreaks or prevent transmission in high-risk areas.

FUNDING

Centers for Disease Control and Prevention and UNICEF.

Combinations of Quality and Frequency of Immunization Activities to Stop and Prevent Poliovirus Transmission in the High-Risk Area of Northwest Nigeria

Background

Frequent supplemental immunization activities (SIAs) with the oral poliovirus vaccine (OPV) represent the primary strategy to interrupt poliovirus transmission in the last endemic areas.

Materials and Methods

Using a differential-equation based poliovirus transmission model tailored to high-risk areas in Nigeria, we perform one-way and multi-way sensitivity analyses to demonstrate the impact of different assumptions about routine immunization (RI) and the frequency and quality of SIAs on population immunity to transmission and persistence or emergence of circulating vaccine-derived polioviruses (cVDPVs) after OPV cessation.

Results

More trivalent OPV use remains critical to avoid serotype 2 cVDPVs. RI schedules with or without inactivated polio vaccine (IPV) could significantly improve population immunity if coverage increases well above current levels in under-vaccinated subpopulations. Similarly, the impact of SIAs on overall population immunity and cVDPV risks depends on their ability to reach under-vaccinated groups (i.e., SIA quality). Lower SIA coverage in the under-vaccinated subpopulation results in a higher frequency of SIAs needed to maintain high enough population immunity to avoid cVDPVs after OPV cessation.

Conclusions

National immunization program managers in northwest Nigeria should recognize the benefits of increasing RI and SIA quality. Sufficiently improving RI coverage and improving SIA quality will reduce the frequency of SIAs required to stop and prevent future poliovirus transmission. Better information about the incremental costs to identify and reach under-vaccinated children would help determine the optimal balance between spending to increase SIA and RI quality and spending to increase SIA frequency.

Modeling options to manage type 1 wild poliovirus imported into Israel in 2013

BACKGROUND

After 25 years without poliomyelitis cases caused by circulating wild poliovirus (WPV) in Israel, sewage sampling detected WPV type 1 (WPV1) in April 2013, despite high vaccination coverage with only inactivated poliovirus vaccine (IPV) since 2005.

METHODS

We used a differential equation-based model to simulate the dynamics of poliovirus transmission and population immunity in Israel due to past exposure to WPV and use of oral poliovirus vaccine (OPV) in addition to IPV. We explored the influences of various immunization options to stop imported WPV1 circulation in Israel.

RESULTS

We successfully modeled the potential for WPVs to circulate without detected cases in Israel. Maintaining a sequential IPV/OPV schedule instead of switching to an IPV-only schedule in 2005 would have kept population immunity high enough in Israel to prevent WPV1 circulation. The Israeli response to WPV1 detection prevented paralytic cases; a more rapid response might have interrupted transmission more quickly.

CONCLUSIONS

IPV-based protection alone might not provide sufficient population immunity to prevent poliovirus transmission after an importation. As countries transition to IPV in immunization schedules, they may need to actively manage population immunity and consider continued use of OPV, to avoid the potential circulation of imported live polioviruses before globally coordinated cessation of OPV use.

Modeling undetected live poliovirus circulation after apparent interruption of transmission: implications for surveillance and vaccination

BACKGROUND

Most poliovirus infections occur with no symptoms and this leads to the possibility of silent circulation, which complicates the confirmation of global goals to permanently end poliovirus transmission. Previous simple models based on hypothetical populations assumed perfect detection of symptomatic cases and suggested the need to observe no paralytic cases from wild polioviruses (WPVs) for approximately 3-4 years to achieve 95% confidence about eradication, but the complexities in real populations and the imperfect nature of surveillance require consideration.

METHODS

We revisit the probability of undetected poliovirus circulation using a more comprehensive model that reflects the conditions in a number of places with different characteristics related to WPV transmission, and we model the actual environmental WPV detection that occurred in Israel in 2013. We consider the analogous potential for undetected transmission of circulating vaccine-derived polioviruses. The model explicitly accounts for the impact of different vaccination activities before and after the last detected case of paralytic polio, different levels of surveillance, variability in transmissibility and neurovirulence among serotypes, and the possibility of asymptomatic participation in transmission by previously-vaccinated or infected individuals.

RESULTS

We find that prolonged circulation in the absence of cases and thus undetectable by case-based surveillance may occur if vaccination keeps population immunity close to but not over the threshold required for the interruption of transmission, as may occur in northwestern Nigeria for serotype 2 circulating vaccine-derived poliovirus in the event of insufficient tOPV use. Participation of IPV-vaccinated individuals in asymptomatic fecal-oral transmission may also contribute to extended transmission undetectable by case-based surveillance, as occurred in Israel. We also find that gaps or quality issues in surveillance could significantly reduce confidence about actual disruption. Maintaining high population immunity and high-quality surveillance for several years after the last detected polio cases will remain critical elements of the polio end game.

CONCLUSIONS

Countries will need to maintain vigilance in their surveillance for polioviruses and recognize that their risks of undetected circulation may differ as a function of their efforts to manage population immunity and to identify cases or circulating live polioviruses.

Modeling undetected live poliovirus circulation after apparent interruption of transmission: implications for surveillance and vaccination

BACKGROUND

Most poliovirus infections occur with no symptoms and this leads to the possibility of silent circulation, which complicates the confirmation of global goals to permanently end poliovirus transmission. Previous simple models based on hypothetical populations assumed perfect detection of symptomatic cases and suggested the need to observe no paralytic cases from wild polioviruses (WPVs) for approximately 3-4 years to achieve 95% confidence about eradication, but the complexities in real populations and the imperfect nature of surveillance require consideration.

METHODS

We revisit the probability of undetected poliovirus circulation using a more comprehensive model that reflects the conditions in a number of places with different characteristics related to WPV transmission, and we model the actual environmental WPV detection that occurred in Israel in 2013. We consider the analogous potential for undetected transmission of circulating vaccine-derived polioviruses. The model explicitly accounts for the impact of different vaccination activities before and after the last detected case of paralytic polio, different levels of surveillance, variability in transmissibility and neurovirulence among serotypes, and the possibility of asymptomatic participation in transmission by previously-vaccinated or infected individuals.

RESULTS

We find that prolonged circulation in the absence of cases and thus undetectable by case-based surveillance may occur if vaccination keeps population immunity close to but not over the threshold required for the interruption of transmission, as may occur in northwestern Nigeria for serotype 2 circulating vaccine-derived poliovirus in the event of insufficient tOPV use. Participation of IPV-vaccinated individuals in asymptomatic fecal-oral transmission may also contribute to extended transmission undetectable by case-based surveillance, as occurred in Israel. We also find that gaps or quality issues in surveillance could significantly reduce confidence about actual disruption. Maintaining high population immunity and high-quality surveillance for several years after the last detected polio cases will remain critical elements of the polio end game.

CONCLUSIONS

Countries will need to maintain vigilance in their surveillance for polioviruses and recognize that their risks of undetected circulation may differ as a function of their efforts to manage population immunity and to identify cases or circulating live polioviruses.

Cluster lot quality assurance sampling: effect of increasing the number of clusters on classification precision and operational feasibility

BACKGROUND

To assess the quality of supplementary immunization activities (SIAs), the Global Polio Eradication Initiative (GPEI) has used cluster lot quality assurance sampling (C-LQAS) methods since 2009. However, since the inception of C-LQAS, questions have been raised about the optimal balance between operational feasibility and precision of classification of lots to identify areas with low SIA quality that require corrective programmatic action.

METHODS

To determine if an increased precision in classification would result in differential programmatic decision making, we conducted a pilot evaluation in 4 local government areas (LGAs) in Nigeria with an expanded LQAS sample size of 16 clusters (instead of the standard 6 clusters) of 10 subjects each.

RESULTS

The results showed greater heterogeneity between clusters than the assumed standard deviation of 10%, ranging from 12% to 23%. Comparing the distribution of 4-outcome classifications obtained from all possible combinations of 6-cluster subsamples to the observed classification of the 16-cluster sample, we obtained an exact match in classification in 56% to 85% of instances.

CONCLUSIONS

We concluded that the 6-cluster C-LQAS provides acceptable classification precision for programmatic action. Considering the greater resources required to implement an expanded C-LQAS, the improvement in precision was deemed insufficient to warrant the effort.

Individual-based modeling of potential poliovirus transmission in connected religious communities in North America with low uptake of vaccination

BACKGROUND

Pockets of undervaccinated individuals continue to raise concerns about their potential to sustain epidemic transmission of vaccine-preventable diseases. Prior importations of live polioviruses (LPVs) into Amish communities in North America led to their recognition as a potential and identifiable linked network of undervaccinated individuals.

METHODS

We developed an individual-based model to explore the potential transmission of a LPV throughout the North American Amish population.

RESULTS

Our model demonstrates the expected limited impact associated with the historical importations, which occurred in isolated communities during the low season for poliovirus transmission. We show that some conditions could potentially lead to wider circulation of LPVs and cases of paralytic polio in Amish communities if an importation occurred during or after 2013. The impact will depend on the uncertain historical immunity to poliovirus infection among members of the community.

CONCLUSIONS

Heterogeneity in immunization coverage represents a risk factor for potential outbreaks of polio if introduction of a LPV occurs, although overall high population immunity in North America suggests that transmission would remain relatively limited. Efforts to prevent spread between Amish church districts with any feasible measures may offer the best opportunity to contain an outbreak and limit its size.

Progress toward global interruption of wild poliovirus transmission, 2010-2013, and tackling the challenges to complete eradication

Despite substantial progress, global polio eradication has remained elusive. Indigenous wild poliovirus (WPV) transmission in 4 endemic countries (Afghanistan, India, Nigeria, and Pakistan) persisted into 2010 and outbreaks from imported WPV continued. By 2013, most outbreaks in the interim were promptly controlled. The number of polio-affected districts globally has declined by 74% (from 481 in 2009 to 126 in 2013), including a 79% decrease in the number of affected districts in endemic countries (from 304 to 63). India is now polio-free. The challenges to success in the remaining polio-endemic countries include (1) threats to the security of vaccinators in each country and a ban on polio vaccination in areas of Afghanistan and Pakistan; (2) a risk of decreased government commitment; and (3) remaining surveillance gaps. Coordinated efforts under the International Health Regulations and efforts to mitigate the challenges provide a clear opportunity to soon secure global eradication.

Detection of vaccine-derived polioviruses in Mexico using environmental surveillance

BACKGROUND

Early detection and control of vaccine-derived poliovirus (VDPV) emergences are essential to secure the gains of polio eradication.

METHODS

Serial sewage samples were collected in 4 towns of Mexico before, throughout, and after the May 2010 oral poliovirus vaccine (OPV) mass immunization campaign. Isolation and molecular analysis of polioviruses from sewage specimens monitored the duration of vaccine-related strains in the environment and emergence of vaccine-derived polioviruses in a population partially immunized with inactivated poliovirus vaccine (IPV).

RESULTS

Sabin strains were identified up to 5-8 weeks after the campaign in all towns; in Aguascalientes, 1 Sabin 3 was isolated 16 weeks after the campaign, following 7 weeks with no Sabin strains detected. In Tuxtla Gutiérrez, type 2 VDPV was isolated from 4 samples collected before and during the campaign, and type 1 VDPV from 1 sample collected 19 weeks afterward. During 2009-2010, coverage in 4 OPV campaigns conducted averaged only 57% and surveillance for acute flaccid paralysis (AFP) was suboptimal (AFP rate<1 per 100,000 population<15 years of age) in Tuxtla Gutierrez.

CONCLUSIONS

VDPVs may emerge and spread in settings with inadequate coverage with IPV/OPV vaccination. Environmental surveillance can facilitate early detection in these settings.

The potential impact of expanding target age groups for polio immunization campaigns

BACKGROUND

Global efforts to eradicate wild polioviruses (WPVs) continue to face challenges due to uninterrupted endemic WPV transmission in three countries and importation-related outbreaks into previously polio-free countries. We explore the potential role of including older children and adults in supplemental immunization activities (SIAs) to more rapidly increase population immunity and prevent or stop transmission.

METHODS

We use a differential equation-based dynamic poliovirus transmission model to analyze the epidemiological impact and vaccine resource implications of expanding target age groups in SIAs. We explore the use of older age groups in SIAs for three situations: alternative responses to the 2010 outbreak in Tajikistan, retrospective examination of elimination in two high-risk states in northern India, and prospective and retrospective strategies to accelerate elimination in endemic northwestern Nigeria. Our model recognizes the ability of individuals with waned mucosal immunity (i.e., immunity from a historical live poliovirus infection) to become re-infected and contribute to transmission to a limited extent.

RESULTS

SIAs involving expanded age groups reduce overall caseloads, decrease transmission, and generally lead to a small reduction in the time to achieve WPV elimination. Analysis of preventive expanded age group SIAs in Tajikistan or prior to type-specific surges in incidence in high-risk areas of India and Nigeria showed the greatest potential benefits of expanded age groups. Analysis of expanded age group SIAs in outbreak situations or to accelerate the interruption of endemic transmission showed relatively less benefit, largely due to the circulation of WPV reaching individuals sooner or more effectively than the SIAs. The India and Nigeria results depend strongly on how well SIAs involving expanded age groups reach relatively isolated subpopulations that sustain clusters of susceptible children, which we assume play a key role in persistent endemic WPV transmission in these areas.

CONCLUSIONS

This study suggests the need to carefully consider the epidemiological situation in the context of decisions to use expanded age group SIAs. Subpopulations of susceptible individuals may independently sustain transmission, which will reduce the overall benefits associated with using expanded age group SIAs to increase population immunity to a sufficiently high level to stop transmission and reduce the incidence of paralytic cases.

Poliovirus vaccination options for achieving eradication and securing the endgame

In 1988, the World Health Assembly resolved to globally eradicate poliomyelitis. As part of a four-pronged strategy with establishment of enhanced surveillance, institution of national immunization days, strengthening routine immunization, and carrying-out mopping-up activities, oral poliovirus vaccine (OPV) was selected as the vaccine-of-choice for eradication. Massive OPV use decreased the number of polio-endemic countries from >125 countries in 1988 to only 3 in 2012 and led to a >99.9% decrease in polio incidence in the corresponding period. In this communication, we will discuss polio vaccination options to accelerate eradication, to mitigate the risks during the planned withdrawal of type 2 OPV, and to secure eradication for future generations.

Review and assessment of poliovirus immunity and transmission: synthesis of knowledge gaps and identification of research needs

With the intensifying global efforts to eradicate wild polioviruses, policymakers face complex decisions related to achieving eradication and managing posteradication risks. These decisions and the expanding use of inactivated poliovirus vaccine (IPV) trigger renewed interest in poliovirus immunity, particularly the role of mucosal immunity in the transmission of polioviruses. Sustained high population immunity to poliovirus transmission represents a key prerequisite to eradication, but poliovirus immunity and transmission remain poorly understood despite decades of studies. In April 2010, the U.S. Centers for Disease Control and Prevention convened an international group of experts on poliovirus immunology and virology to review the literature relevant for modeling poliovirus transmission, develop a consensus about related uncertainties, and identify research needs. This article synthesizes the quantitative assessments and research needs identified during the process. Limitations in the evidence from oral poliovirus vaccine (OPV) challenge studies and other relevant data led to differences in expert assessments, indicating the need for additional data, particularly in several priority areas for research: (1) the ability of IPV-induced immunity to prevent or reduce excretion and affect transmission, (2) the impact of waning immunity on the probability and extent of poliovirus excretion, (3) the relationship between the concentration of poliovirus excreted and infectiousness to others in different settings, and (4) the relative role of fecal-oral versus oropharyngeal transmission. This assessment of current knowledge supports the immediate conduct of additional studies to address the gaps.

Characterizing poliovirus transmission and evolution: insights from modeling experiences with wild and vaccine-related polioviruses

With national and global health policymakers facing numerous complex decisions related to achieving and maintaining polio eradication, we expanded our previously developed dynamic poliovirus transmission model using information from an expert literature review process and including additional immunity states and the evolution of oral poliovirus vaccine (OPV). The model explicitly considers serotype differences and distinguishes fecal-oral and oropharyngeal transmission. We evaluated the model by simulating diverse historical experiences with polioviruses, including one country that eliminated wild poliovirus using both OPV and inactivated poliovirus vaccine (IPV) (USA), three importation outbreaks of wild poliovirus (Albania, the Netherlands, Tajikistan), one situation in which no circulating vaccine-derived polioviruses (cVDPVs) emerge despite annual OPV use and cessation (Cuba), three cVDPV outbreaks (Haiti, Madura Island in Indonesia, northern Nigeria), one area of current endemic circulation of all three serotypes (northern Nigeria), and one area with recent endemic circulation and subsequent elimination of multiple serotypes (northern India). We find that when sufficient information about the conditions exists, the model can reproduce the general behavior of poliovirus transmission and outbreaks while maintaining consistency in the generic model inputs. The assumption of spatially homogeneous mixing remains a significant limitation that affects the performance of the differential equation-based model when significant heterogeneities in immunity and mixing may exist. Further studies on OPV virus evolution and improved understanding of the mechanisms of mixing and transmission may help to better characterize poliovirus transmission in populations. Broad application of the model promises to offer insights in the context of global and national policy and economic models.

Expert review on poliovirus immunity and transmission

Successfully managing risks to achieve wild polioviruses (WPVs) eradication and address the complexities of oral poliovirus vaccine (OPV) cessation to stop all cases of paralytic poliomyelitis depends strongly on our collective understanding of poliovirus immunity and transmission. With increased shifting from OPV to inactivated poliovirus vaccine (IPV), numerous risk management choices motivate the need to understand the tradeoffs and uncertainties and to develop models to help inform decisions. The U.S. Centers for Disease Control and Prevention hosted a meeting of international experts in April 2010 to review the available literature relevant to poliovirus immunity and transmission. This expert review evaluates 66 OPV challenge studies and other evidence to support the development of quantitative models of poliovirus transmission and potential outbreaks. This review focuses on characterization of immunity as a function of exposure history in terms of susceptibility to excretion, duration of excretion, and concentration of excreted virus. We also discuss the evidence of waning of host immunity to poliovirus transmission, the relationship between the concentration of poliovirus excreted and infectiousness, the importance of different transmission routes, and the differences in transmissibility between OPV and WPV. We discuss the limitations of the available evidence for use in polio risk models, and conclude that despite the relatively large number of studies on immunity, very limited data exist to directly support quantification of model inputs related to transmission. Given the limitations in the evidence, we identify the need for expert input to derive quantitative model inputs from the existing data.

Oral poliovirus vaccine evolution and insights relevant to modeling the risks of circulating vaccine-derived polioviruses (cVDPVs)

The live, attenuated oral poliovirus vaccine (OPV) provides a powerful tool for controlling and stopping the transmission of wild polioviruses (WPVs), although the risks of vaccine-associated paralytic polio (VAPP) and circulating vaccine-derived poliovirus (cVDPV) outbreaks exist as long as OPV remains in use. Understanding the dynamics of cVDPV emergence and outbreaks as a function of population immunity and other risk factors may help to improve risk management and the development of strategies to respond to possible outbreaks. We performed a comprehensive review of the literature related to the process of OPV evolution and information available from actual experiences with cVDPV outbreaks. Only a relatively small fraction of poliovirus infections cause symptoms, which makes direct observation of the trajectory of OPV evolution within a population impractical and leads to significant uncertainty. Despite a large global surveillance system, the existing genetic sequence data largely provide information about transmitted virulent polioviruses that caused acute flaccid paralysis, and essentially no data track the changes that occur in OPV sequences as the viruses transmit largely asymptomatically through real populations with suboptimal immunity. We updated estimates of cVDPV risks based on actual experiences and identified the many limitations in the existing data on poliovirus transmission and immunity and OPV virus evolution that complicate modeling. Modelers should explore the space of potential model formulations and inputs consistent with the available evidence and future studies should seek to improve our understanding of the OPV virus evolution process to provide better information for policymakers working to manage cVDPV risks.

Trends in the risk of U.S. polio outbreaks and poliovirus vaccine availability for response

OBJECTIVES

The United States eliminated indigenous wild polioviruses (WPVs) in 1979 and switched to inactivated poliovirus vaccine in 2000, which quickly ended all indigenous live poliovirus transmission. Continued WPV circulation and use of oral poliovirus vaccine globally allow for the possibility of reintroduction of these viruses. We evaluated the risk of a U.S. polio outbreak and explored potential vaccine needs for outbreak response.

METHODS

We synthesized information available on vaccine coverage, exemptor populations, and population immunity. We used an infection transmission model to explore the potential dynamics of a U.S. polio outbreak and potential vaccine needs for outbreak response, and assessed the impacts of heterogeneity in population immunity for two different subpopulations with potentially low coverage.

RESULTS

Although the risk of poliovirus introduction remains real, widespread transmission of polioviruses appears unlikely in the U.S., given high routine coverage. However, clusters of un- or underimmunized children might create pockets of susceptibility that could potentially lead to one or more paralytic polio cases. We found that the shift toward combination vaccine utilization, with limited age indications for use, and other current trends (e.g., decreasing proportion of the population with immunity induced by live polioviruses and aging of vaccine exemptor populations) might increase the vulnerability to poliovirus reintroduction at the same time that the ability to respond may decrease.

CONCLUSIONS

The U.S. poliovirus vaccine stockpile remains an important resource that may potentially be needed in the future to respond to an outbreak if a live poliovirus gets imported into a subpopulation with low vaccination coverage.

Epidemiology of pertussis in a West African community before and after introduction of a widespread vaccination program

The control of pertussis remains a worldwide concern. Little has been documented about its epidemiology in Africa. The authors have studied pertussis in a prospective cohort of children in a rural West African community over a 13-year period comprising time before and after introduction of a vaccination program. Children under age 15 years who were residents of the Niakhar study area in Senegal were followed prospectively between January 1984 and December 1996 for the occurrence of pertussis. Morbidity and mortality rates were extremely high before the launch of immunization. Crude incidence was 183 per 1,000 child-years at risk under age 5 years, with a 2.8% case-fatality rate. After the introduction of the vaccination program, overall incidence dropped rapidly and dramatically-by 27% after 3 years and 46% after 6 years. The decline in incidence involved all age groups but was most substantial in the group under age 5 years and was particularly pronounced in unvaccinated infants. The median age of acquisition of the disease rose steadily with population vaccine coverage. This study shows the tremendous magnitude of the disease burden in children and the rapid decline after vaccination, and it suggests a strong herd-immunity effect.