Progress Toward Poliomyelitis Eradication - Worldwide, January 2022-December 2023

In 1988, poliomyelitis (polio) was targeted for eradication. Global efforts have led to the eradication of two of the three wild poliovirus (WPV) serotypes (types 2 and 3), with only WPV type 1 (WPV1) remaining endemic, and only in Afghanistan and Pakistan. This report describes global polio immunization, surveillance activities, and poliovirus epidemiology during January 2022-December 2023, using data current as of April 10, 2024. In 2023, Afghanistan and Pakistan identified 12 total WPV1 polio cases, compared with 22 in 2022. WPV1 transmission was detected through systematic testing for poliovirus in sewage samples (environmental surveillance) in 13 provinces in Afghanistan and Pakistan, compared with seven provinces in 2022. The number of polio cases caused by circulating vaccine-derived polioviruses (cVDPVs; circulating vaccine virus strains that have reverted to neurovirulence) decreased from 881 in 2022 to 524 in 2023; cVDPV outbreaks (defined as either a cVDPV case with evidence of circulation or at least two positive environmental surveillance isolates) occurred in 32 countries in 2023, including eight that did not experience a cVDPV outbreak in 2022. Despite reductions in paralytic polio cases from 2022, cVDPV cases and WPV1 cases (in countries with endemic transmission) were more geographically widespread in 2023. Renewed efforts to vaccinate persistently missed children in countries and territories where WPV1 transmission is endemic, strengthen routine immunization programs in countries at high risk for poliovirus transmission, and provide more effective cVDPV outbreak responses are necessary to further progress toward global polio eradication.

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.

Global oral poliovirus vaccine stockpile management as an essential preparedness and response mechanism for type 2 poliovirus outbreaks following global oral poliovirus vaccine type 2 withdrawal

Following the global declaration of indigenous wild poliovirus type 2 eradication in 2015, the world switched to oral polio vaccine (OPV) that removed the type 2 component. This 'switch' included the widespread introduction of inactivated poliovirus vaccine and the creation of a stockpile of monovalent type 2 OPV (mOPV2) to respond to potential polio virus Type 2 (PV2) outbreaks and events. With subsequent detection of outbreaks of circulating vaccine-derived poliovirus type 2 (cVDPV2), it was necessary to use this stockpile for outbreak response. Not only were more outbreaks detected than anticipated in the first few years after the switch, but the number of supplemental immunization activities (SIAs) used to stop transmission was often high, and in many cases did not stop wider transmission. Use of mOPV type 2 led in some locations to the emergence of new outbreaks that required further use of the vaccine from the stockpile. In the following years, stockpile management became a critical element of the cVDPV2 outbreak response strategy and continued to evolve to include trivalent OPV and genetically stabilized 'novel OPV type 2' vaccines in the stockpile. An overview of this process and its evolution is presented to highlight several of these management challenges. The unpredictable vaccine demand, fixed production and procurement timelines, resource requirements, and multiple vaccine types contributes to the complexity of assuring appropriate vaccine availability for this critical programmatic need to stop outbreaks.

Modelling the spread of serotype-2 vaccine derived-poliovirus outbreak in Pakistan and Afghanistan to inform outbreak control strategies in the context of the COVID-19 pandemic

Background

Since July 2019, Pakistan and Afghanistan have been facing an outbreak of serotype-2 circulating vaccine derived poliovirus (cVDPV2) in addition to continued transmission of serotype-1 wild poliovirus (WPV1) and SARS-CoV-2 in 2020. Understanding the risks of cVDPV2 transmission due to pause of global vaccination efforts and the impact of potential vaccination response strategies in the current context of COVID-19 mitigation measures is critical.

Methods

We developed a stochastic, geographically structured mathematical model of cVDPV2 transmission which captures both mucosal and humoral immunity separately and allows for reversion of serotype-2 oral polio vaccine (OPV2) virus to cVDPV2 following vaccine administration. The model includes geographic heterogeneities in vaccination coverage, population immunity and population movement. The model was fitted to historic cVDPV2 cases in Pakistan and Afghanistan between January 2010-April 2016 and July 2019-March 2020 using iterated particle filtering. The model was used to simulate spread of cVDPV2 infection from July 2019 to explore impact of various proposed vaccination responses on stopping transmission and risk of spread of reverted Sabin-2 under varying assumptions of impacts from COVID-19 lockdown measures on movement patterns as well as declines in vaccination coverage.

Results

Simulated monthly incidence of cVDPV2 from the best-fit model demonstrated general spatio-temporal alignment with observed cVDPV2 cases. The model predicted substantial spread of cVDPV2 infection, with widespread transmission through 2020 in the absence of any vaccination activities. Vaccination responses were predicted to substantially reduce transmission and case burden, with a greater impact from earlier responses and those with larger geographic scope. While the greatest risk of seeding reverted Sabin-2 was predicted in areas targeted with OPV2, subsequent spread was greatest in areas with no or delayed response. The proposed vaccination strategy demonstrated ability to stop the cVDPV2 outbreak (with low risk of reverted Sabin-2 spread) by February 2021.

Conclusion

Outbreak response vaccination campaigns against cVDPV2 will be challenging throughout the COVID-19 pandemic but must be implemented urgently when feasible to stop transmission of cVDPV2.

Progress toward polio eradication - worldwide, 2014-2015

In 1988, the World Health Assembly of the World Health Organization (WHO) resolved to eradicate polio worldwide. Wild poliovirus (WPV) transmission has been interrupted in all but three countries (Afghanistan, Nigeria, and Pakistan). No WPV type 2 cases have been detected worldwide since 1999, and the last WPV type 3 case was detected in Nigeria in November 2012; since 2012, only WPV type 1 has been detected. Circulating vaccine-derived poliovirus (cVDPV), usually type 2, continues to cause cases of paralytic polio in communities with low population immunity. In 2012, the World Health Assembly declared global polio eradication "a programmatic emergency for global public health", and in 2014, WHO declared the international spread of WPV to previously polio-free countries to be "a public health emergency of international concern". This report summarizes global progress toward polio eradication during 2014-2015 and updates previous reports. In 2014, a total of 359 WPV cases were reported in nine countries worldwide. Although reported WPV cases increased in Pakistan and Afghanistan, cases in Nigeria decreased substantially in 2014, and encouraging progress toward global WPV transmission interruption has occurred. Overcoming ongoing challenges to interruption of WPV transmission globally will require sustained programmatic enhancements, including improving the quality of supplementary immunization activities (SIAs) to interrupt transmission in Afghanistan and Pakistan and to prevent WPV exportation to polio-free countries.

Lot quality assurance sampling to monitor supplemental immunization activity quality: an essential tool for improving performance in polio endemic countries

Monitoring the quality of supplementary immunization activities (SIAs) is a key tool for polio eradication. Regular monitoring data, however, are often unreliable, showing high coverage levels in virtually all areas, including those with ongoing virus circulation. To address this challenge, lot quality assurance sampling (LQAS) was introduced in 2009 as an additional tool to monitor SIA quality. Now used in 8 countries, LQAS provides a number of programmatic benefits: identifying areas of weak coverage quality with statistical reliability, differentiating areas of varying coverage with greater precision, and allowing for trend analysis of campaign quality. LQAS also accommodates changes to survey format, interpretation thresholds, evaluations of sample size, and data collection through mobile phones to improve timeliness of reporting and allow for visualization of campaign quality. LQAS becomes increasingly important to address remaining gaps in SIA quality and help focus resources on high-risk areas to prevent the continued transmission of wild poliovirus.

Polio eradication initiative in Afghanistan, 1997-2013

BACKGROUND

This article reviews the epidemiology of polio, acute flaccid paralysis (AFP) surveillance, and the implementation of supplemental immunization activities (SIAs) in Afghanistan from 1997 thru 2013.

METHODS

Published reports and unpublished national data on polio cases, AFP surveillance, and SIAs were analyzed. Recommendations from independent advisory groups and Afghan government informed the conclusions.

RESULTS

From 1997 thru 2013, the annual number of confirmed polio cases fluctuated from a low of 4 in 2004 to a high of 80 in 2011. Wild poliovirus types 2 and 3 were last reported in 1997 and 2010, respectively. Circulating vaccine-derived poliovirus type 2 emerged in 2009. AFP surveillance quality in children aged <15 years improved over time, achieving rates>8 per 100,000 population. Since 2001, at least 6 SIAs have been conducted annually.

CONCLUSIONS

Afghanistan has made progress moving closer to eliminating polio. The program struggles to reach all children because of management and accountability problems in the field, inaccessible populations, and inadequate social mobilization. Consequently, too many children are missed during SIAs. Afghanistan adopted a national emergency action plan in 2012 to address these issues, but national elimination will require consistent and complete implementation of proven strategies.

Progress toward poliomyelitis eradication--Afghanistan and Pakistan, January 2013-August 2014

In 2012, the World Health Assembly declared the completion of polio eradication a programmatic emergency for global public health and called for a comprehensive polio endgame strategy. Afghanistan and Pakistan are two of the three remaining countries (the other is Nigeria) where circulation of indigenous wild poliovirus (WPV) has never been interrupted. This report updates previous reports and describes polio eradication activities and progress in Afghanistan and Pakistan during January 2013-August 2014. In Afghanistan, 14 WPV cases were reported in 2013, compared with 37 cases in 2012; nine cases were reported during January-August 2014, compared with six cases during the same period in 2013. In Pakistan, 93 WPV cases were reported in 2013, compared with 58 cases in 2012; 170 cases were reported during January-August 2014, compared with 33 cases during the same period in 2013. All WPV cases reported during January 2013-August 2014 were WPV type 1 (WPV1). Vaccination campaigns have been banned since June 2012 in specific areas in Pakistan, where an estimated 300,000 children aged <5 years reside and where 69% of WPV cases have occurred in 2014. To accomplish the objectives of the Polio Eradication and Endgame Strategic Plan for 2013-2018 both countries should continue to negotiate access of vaccinators to insecure and temporarily inaccessible areas, improve immunization program performance to reach more children in accessible areas, and ensure that political and health leaders at all levels are fully committed to the program, including being committed to providing financial resources needed to fully implement all the recommendations of external technical advisory groups. Both countries should also continue to strengthen cross-border collaboration to improve surveillance and case detection, coordinate outbreak response, and maximize vaccination coverage of children moving between the two countries.

The effect of mass immunisation campaigns and new oral poliovirus vaccines on the incidence of poliomyelitis in Pakistan and Afghanistan, 2001-11: a retrospective analysis

BACKGROUND

Pakistan and Afghanistan are two of the three remaining countries yet to interrupt wild-type poliovirus transmission. The increasing incidence of poliomyelitis in these countries during 2010-11 led the Executive Board of WHO in January, 2012, to declare polio eradication a "programmatic emergency for global public health". We aimed to establish why incidence is rising in these countries despite programme innovations including the introduction of new vaccines.

METHODS

We did a matched case-control analysis based on a database of 46,977 children aged 0-14 years with onset of acute flaccid paralysis between Jan 1, 2001, and Dec 31, 2011. The vaccination history of children with poliomyelitis was compared with that of children with acute flaccid paralysis due to other causes to estimate the clinical effectiveness of oral poliovirus vaccines (OPVs) in Afghanistan and Pakistan by conditional logistic regression. We estimated vaccine coverage and serotype-specific vaccine-induced population immunity in children aged 0-2 years and assessed their association with the incidence of poliomyelitis over time in seven regions of Afghanistan and Pakistan.

FINDINGS

Between Jan 1, 2001, and Dec 31, 2011, there were 883 cases of serotype 1 poliomyelitis (710 in Pakistan and 173 in Afghanistan) and 272 cases of poliomyelitis serotype 3 (216 in Pakistan and 56 in Afghanistan). The estimated clinical effectiveness of a dose of trivalent OPV against serotype 1 poliomyelitis was 12·5% (95% CI 5·6-18·8) compared with 34·5% (16·1-48·9) for monovalent OPV (p=0·007) and 23·4% (10·4-34·6) for bivalent OPV (p=0·067). Bivalent OPV was non-inferior compared with monovalent OPV (p=0·21). Vaccination coverage decreased during 2006-11 in the Federally Administered Tribal Areas (FATA), Balochistan, and Khyber Pakhtunkhwa in Pakistan and in southern Afghanistan. Although partially mitigated by the use of more effective vaccines, these decreases in coverage resulted in lower vaccine-induced population immunity to poliovirus serotype 1 in FATA and Balochistan and associated increases in the incidence of poliomyelitis.

INTERPRETATION

The effectiveness of bivalent OPV is comparable with monovalent OPV and can therefore be used in eradicating serotype 1 poliomyelitis whilst minimising the risks of serotype 3 outbreaks. However, decreases in vaccination coverage in parts of Pakistan and southern Afghanistan have severely limited the effect of this vaccine.

FUNDING

Poliovirus Research subcommittee of WHO, Royal Society, and Medical Research Council.

Lesion Explorer: a comprehensive segmentation and parcellation package to obtain regional volumetrics for subcortical hyperintensities and intracranial tissue

Subcortical hyperintensities (SH) are a commonly observed phenomenon on MRI of the aging brain (Kertesz et al., 1988). Conflicting behavioral, cognitive and pathological associations reported in the literature underline the need to develop an intracranial volumetric analysis technique to elucidate pathophysiological origins of SH in Alzheimer's disease (AD), vascular cognitive impairment (VCI) and normal aging (De Leeuw et al., 2001; Mayer and Kier, 1991; Pantoni and Garcia, 1997; Sachdev et al., 2008). The challenge is to develop processing tools that effectively and reliably quantify subcortical small vessel disease in the context of brain tissue compartments. Segmentation and brain region parcellation should account for SH subtypes which are often classified as: periventricular (pvSH) and deep white (dwSH), incidental white matter disease or lacunar infarcts and Virchow-Robin spaces. Lesion Explorer (LE) was developed as the final component of a comprehensive volumetric segmentation and parcellation image processing stream built upon previously published methods (Dade et al., 2004; Kovacevic et al., 2002). Inter-rater and inter-method reliability was accomplished both globally and regionally. Volumetric analysis showed high inter-rater reliability both globally (ICC=.99) and regionally (ICC=.98). Pixel-wise spatial congruence was also high (SI=.97). Whole brain pvSH volumes yielded high inter-rater reliability (ICC=.99). Volumetric analysis against an alternative kNN segmentation revealed high inter-method reliability (ICC=.97). Comparison with visual rating scales showed high significant correlations (ARWMC: r=.86; CHIPS: r=.87). The pipeline yields a comprehensive and reliable individualized volumetric profile for subcortical vasculopathy that includes regionalized (26 brain regions) measures for: GM, WM, sCSF, vCSF, lacunar and non-lacunar pvSH and dwSH.

Prevalence of hepatitis B among Afghan refugees living in Balochistan, Pakistan

OBJECTIVE

Continued civil war and political instability in Afghanistan have lead to a huge influx of refugees into the neighboring provinces in Pakistan. This study was conducted to estimate seroprevalence of hepatitis B and to identify potential risk factors for hepatitis B virus (HBV) transmission among the refugees living in the camps of Balochistan Province, Pakistan.

METHODS

A cross-sectional survey of hepatitis B surface antigen (HBsAg) was conducted during October 2003. We obtained the registration list to select families randomly from the refugee camps. A husband, wife and one of their children, selected at random, were enrolled in the study. Study subjects with positive laboratory results for HBsAg were compared with those who were negative for HBsAg.

RESULTS

Field workers interviewed 301 families with a total of 903 study subjects. Blood specimens of 75 study subjects (8.3%, 95% CI 6.6-10.3) were positive for HBsAg. There were 37 husbands (12.3%, 95% CI 7.2-14.4) and 21 wives (7.0%, 95% CI 4.5-10.6) positive for HBsAg. Out of 301 children, 17 (5.6%, 95% CI 3.4-9.1) were positive for HBsAg. Receiving more than ten injections during the previous year increased the risk of HBV infection (OR 3.5, 95% CI 1.8-6.7). A child positive for HBsAg was more likely to have a positive parent compared to an HBsAg negative child (OR 5.7, 95% CI 2.0-16.5).

CONCLUSION

Hepatitis B is highly endemic among Afghan refugees living in these camps. Unsafe injection practices will continue to cause a steady increase in the magnitude of this health problem until appropriate control measures are taken. The possibility of mother-to-child transmission underscores the need to include vaccination against hepatitis B as part of routine immunization in this population.

Incremental cost-effectiveness of supplementary immunization activities to prevent neonatal tetanus in Pakistan

OBJECTIVE

This study aimed to estimate the incremental cost-effectiveness of supplementary immunization activities to prevent neonatal tetanus in the Loralai district of Pakistan. The supplemental immunization activities were carried out in two phases during 2001-03.

METHODS

A state-transition model was used to estimate the effect of routine vaccination with tetanus toxoid as well as vaccination with tetanus toxoid during supplementary immunization activities. The model follows each woman in the target population from birth until the end of her childbearing years, using age-specific fertility data and vaccination history to determine the number of births at risk for neonatal tetanus. Recently published data on the incidence of neonatal tetanus from Loralai were used to determine the number of cases occurring with and without supplementary immunization activities. Data on the costs of the activities were collected from the UNICEF office in Balochistan and from the Provincial Health Department.

FINDINGS

Using base-case assumptions we estimated that the supplementary immunization activities would prevent 280 cases of neonatal tetanus and 224 deaths from neonatal tetanus between 2001 and 2034. Implementation of the supplementary activities was relatively inexpensive. The cost per tetanus toxoid dose delivered was 0.40 U.S. dollars. In the base-case analysis the cost per death averted was 117.00 U.S. dollars (95% confidence interval (CI) = 78-205 U.S. dollars) and the cost per disability-adjusted life year (DALY) averted was 3.61 U.S. dollars (95% Cl = 2.43-6.39 U.S. dollars).

CONCLUSION

Compared with similar analyses of other interventions, the cost per DALY averted is a favourable cost-effectiveness ratio. However, if routine diphtheria-tetanus-pertussis vaccination coverage in the Loralai district had been higher (at a coverage rate of about 80%) the cost-effectiveness of the intervention would have been even more favourable, at 2.65 U.S. dollars per DALY averted.

Neonatal tetanus: mortality rate and risk factors in Loralai District, Pakistan

BACKGROUND

This study was conducted to estimate the neonatal tetanus (NNT) mortality rate and to identify the risk factors for NNT deaths in Loralai District, Pakistan.

METHOD

We conducted a community-based cross-sectional survey during July-September 1997. We stratified the sample proportionate to population of union councils. The most populous village in a union council was selected first. We interviewed the women, selected randomly, who had a live birth in the 18 months preceding the survey. We conducted a matched case-control study to identify the risk factors for NNT deaths. We used the World Health Organization criteria to enrol cases, identified during the cross-sectional survey or registered at the district hospital. We enrolled three community-based controls per case, matched on the area of residence, immunization status and date of birth.

RESULTS

Of the 1547 live births, there were 36 neonatal deaths due to tetanus. The NNT mortality rate in the district was 23 per 1000 live births (95% CI: 16-30). For the case-control study, we enrolled 41 cases and 123 controls. Using conditional logistic regression, the risk of NNT death was increased with the use of soil as delivery surface (O.R = 3.2, 95% CI: 1.1-10.2), father's illiteracy (OR = 3.2, 95% CI: 1.3-8.1) and possession of sheep at home (OR = 2, 95% CI: 1.0-5.0). The population attributable risk per cent for soil as delivery surface was 64%.

CONCLUSION

Transmission of infection while using soil as the delivery surface can occur through direct or indirect contamination of the fresh umbilical wound. Use of safer delivery practices in general and clean surfaces in particular should be encouraged to reduce the NNT mortality rate in the area.