A New Method for Estimating the Coverage of Mass Vaccination Campaigns Against Poliomyelitis From Surveillance Data.

Mass vaccination campaigns with the oral poliovirus vaccine targeting children aged <5 years are a critical component of the global poliomyelitis eradication effort. Monitoring the coverage of these campaigns is essential to allow corrective action, but current approaches are limited by their cross-sectional nature, nonrandom sampling, reporting biases, and accessibility issues. We describe a new Bayesian framework using data augmentation and Markov chain Monte Carlo methods to estimate variation in vaccination coverage from children's vaccination histories investigated during surveillance for acute flaccid paralysis. We tested the method using simulated data with at least 200 cases and were able to detect undervaccinated groups if they exceeded 10% of all children and temporal changes in coverage of ±10% with greater than 90% sensitivity. Application of the method to data from Pakistan for 2010-2011 identified undervaccinated groups within the Balochistan/Federally Administered Tribal Areas and Khyber Pakhtunkhwa regions, as well as temporal changes in coverage. The sizes of these groups are consistent with the multiple challenges faced by the program in these regions as a result of conflict and insecurity. Application of this new method to routinely collected data can be a useful tool for identifying poorly performing areas and assisting in eradication efforts.

Mass vaccination: when and why.

With increased demand for smallpox vaccination during the nineteenth century, vaccination days--early mass vaccination campaigns--were conducted over time-limited periods to rapidly and efficiently protect maximum numbers of susceptible persons. Two centuries later, the challenge to rapidly and efficiently protect populations by mass vaccintion continues, despite the strengthening of routine immunization services in many countries through the Expanded Programme on Immunization strategies and GAVI support. Perhaps the most widely accepted reason for mass vaccination is to rapidly increase population (herd) immunity in the setting of an existing or potential outbreak, thereby limiting the morbidity and mortality that might result, especially when there has been no routine vaccination, or because populations have been displaced and routine immunization services disrupted. A second important use of mass vaccination is to accelerate disease control to rapidly increase coverage with a new vaccine at the time of its introduction into routine immunization programmes, and to attain the herd immunity levels required to meet international targets for eradication and mortality reduction. In the twenty-first century, mass vaccination and routine immunization remain a necessary alliance for attaining both national and international goals in the control of vaccine preventable disease.

Eradicating polio: today's challenges and tomorrow's legacy.

Since its launch in 1988, the Global Polio Eradication Initiative has grown into one of the largest international health efforts in history, operating in every country and area in the world. The burden of polio disease has been reduced by over 99%, and the number of countries with indigenous virus has fallen from more than 125 to just four. As importantly, a strong surveillance and laboratory infrastructure has been established for vaccine-preventable diseases (including measles, tetanus, yellow fever, rubella and Japanese encephalitis), and a massive investment has been made in the physical infrastructure and human resources needed to deliver routine immunizations and other health services in developing countries. Between 2000 and 2003, new challenges to polio eradication emerged, threatening the interruption of the transmission of wild poliovirus globally and the eventual elimination of any residual polio disease as the result of the continued use of oral polio vaccines. By the end of 2005, a range of solutions had been developed to address these late challenges, including two new monovalent oral polio vaccines, new and robust international standards for the response to polio outbreaks, and renewed political commitment across the countries that remain infected. As importantly, a comprehensive strategy had been established for managing the long-term risks of paralytic polio, centred, ironically, on the eventual elimination from routine immunizations of the vaccine that is still central to the success of the global eradication effort.

Progress towards global polio eradication.

Twelve years after the global polio eradication goal was set, polio cases have declined by more than 95% world-wide. Polio immunization campaigns have been conducted in every endemic country with as many as 470 million children immunized per year. Intense wild poliovirus transmission is now limited to South Asia and sub-Saharan Africa. To achieve eradication at the earliest possible date, immunization campaigns are being intensified in the remaining endemic countries. Major programmatic challenges include reaching vulnerable children in areas with armed conflict and ensuring full financial and political support for the initiative. With global eradication imminent, WHO is preparing for post-eradication issues: containment of polioviruses, certification of eradication, and stopping immunization.

Poliomyelitis eradication: progress, challenges for the end game, and preparation for the post-eradication era.

In 1988, the World Health Assembly resolved to eradicate poliomyelitis globally by the year 2000. Dramatic progress toward this goal has occurred: three of the six WHO regions (Region of the Americas, European Region, and Western Pacific Region) are now polio free; and the number of polio-endemic countries decreased from over 125 in 1988 to 30 in 1999. Intensified efforts currently are underway to reach the target as soon as possible after 2000 in the three remaining polio-endemic WHO regions (African Region, Eastern Mediterranean Region, and South-East Asia Region). Even in polio-endemic regions, many countries are already polio free as the geographic extent of poliovirus shrinks while others. especially those experiencing conflict and war, pose substantial challenges to implementing the proven polio eradication strategies. Increasing attention and research now are devoted to the certification of polio eradication in the polio-free regions (that will include the first phase of implementing the Global Plan of Action for the laboratory containment of wild poliovirus) and formulating a policy for stopping all polio vaccination once eradication, containment, and global certification have been achieved. This report outlines the progress toward polio eradication and highlights some of the remaining issues and challenges that must be addressed before polio becomes a disease that future generations know only by history.

Possible global strategies for stopping polio vaccination and how they could be harmonized.

One of the challenges of the polio eradication initiative over the next few years will be the formulation of an optimal strategy for stopping poliovirus vaccination after global certification of polio eradication has been accomplished. This strategy must maximize the benefits and minimize the risks. A number of strategies are currently under consideration, including: (i) synchronized global discontinuation of use of oral poliovirus vaccine (OPV); (ii) regional or subregional coordinated OPV discontinuation; and (iii) moving from trivalent to bivalent or monovalent OPV. Other options include moving from OPV to global use of IPV for an interim period before cessation of IPV use (to eliminate circulation of vaccine-derived poliovirus, if necessary) or development of new OPV strains that are not transmissible. Each of these strategies is associated with specific advantages (financial benefits for OPV discontinuation) and disadvantages (cost of switch to IPV) and inherent uncertainties (risk of continued poliovirus circulation in certain populations or prolonged virus replication in immunodeficient persons). An ambitious research agenda addresses the remaining questions and issues. Nevertheless, several generalities are already clear. Unprecedented collaboration between countries, regions, and indeed the entire world will be required to implement a global OPV discontinuation strategy Regulatory approval will be needed for an interim bivalent OPV or for monovalent OPV in many countries. Manufacturers will need sufficient lead time to produce sufficient quantities of IPV Finally, the financial implications for any of these strategies need to be considered. Whatever strategy is followed it will be necessary to stockpile supplies of a poliovirus-containing vaccine (most probably all three types of monovalent OPV), and to develop contingency plans to respond should an outbreak of polio occur after stopping vaccination.

Disease eradication as a public health strategy: a case study of poliomyelitis eradication.

Disease eradication as a public health strategy was discussed at international meetings in 1997 and 1998. In this article, the ongoing poliomyelitis eradication initiative is examined using the criteria for evaluating candidate diseases for eradication proposed at these meetings, which covered costs and benefits, biological determinants of eradicability (technical feasibility) and societal and political considerations (operational feasibility). The benefits of poliomyelitis eradication are shown to include a substantial investment in health services delivery, the elimination of a major cause of disability, and far-reaching intangible effects, such as establishment of a "culture of prevention". The costs are found to be financial and finite, despite some disturbances to the delivery of other health services. The "technical" feasibility of poliomyelitis eradication is seen in the absence of a non-human reservoir and the presence of both an effective intervention and delivery strategy (oral poliovirus vaccine and national immunization days) and a sensitive and specific diagnostic tool (viral culture of specimens from acute flaccid paralysis cases). The certification of poliomyelitis eradication in the Americas in 1994 and interruption of endemic transmission in the Western Pacific since March 1997 confirm the operational feasibility of this goal. When the humanitarian, economic and consequent benefits of this initiative are measured against the costs, a strong argument is made for eradication as a valuable disease control strategy.

Eradication of poliomyelitis in countries affected by conflict.

The global initiative to eradicate poliomyelitis is focusing on a small number of countries in Africa (Angola, Democratic Republic of the Congo, Liberia, Sierra Leone, Somalia, Sudan) and Asia (Afghanistan, Tajikistan), where progress has been hindered by armed conflict. In these countries the disintegration of health systems and difficulties of access are major obstacles to the immunization and surveillance strategies necessary for polio eradication. In such circumstances, eradication requires special endeavours, such as the negotiation of ceasefires and truces and the winning of increased direct involvement by communities. Transmission of poliovirus was interrupted during conflicts in Cambodia, Colombia, El Salvador, Peru, the Philippines, and Sri Lanka. Efforts to achieve eradication in areas of conflict have led to extra health benefits: equity in access to immunization, brought about because every child has to be reached; the revitalization and strengthening of routine immunization services through additional externally provided resources; and the establishment of disease surveillance systems. The goal of polio eradication by the end of 2000 remains attainable if supplementary immunization and surveillance can be accelerated in countries affected by conflict.

Invited commentary: The scientific basis for stopping polio immunization.

PIP: This article summarizes the WHO-sponsored meeting of virologists, immunologists, and epidemiologists in March 1998 to address the final and controversial stage of the polio eradication initiative. The meeting commissioned Fine and Carneiro's literature review and mathematical model delineating the important gaps in the scientific knowledge and helped define the research agenda of the remaining few years of the initiative. Fine and Carneiro proposed that the possibility of continuing circulation of vaccine-derived polioviruses (VDPV) could not be excluded with absolute certainty. They also argued that VDPV may continue to circulate after use of oral polio vaccine stops and that immunodeficient persons may be a potential reservoir from which VDPV could be reintroduced into the general population. Their work further highlights that high-level enterovirus surveillance will be essential in the years after immunization has stopped.^ieng

Poliomyelitis eradication.

Since the poliomyelitis eradication program began in 1988, the number of poliovirus infected continents and countries have decreased from five to two and from greater than 100 to 53, respectively. A nearly 90% reduction in the incidence of polio has been achieved with a corresponding decrease in virus genomic heterogeneity. Major challenges to eradication remain in south Asia and Africa in those areas with hot and humid climates, high population density, and high birth rates. Of particular concern are countries with ongoing social unrest and poor health infrastructure. With the approaching eradication of polio, post-eradication issues are now being addressed. The World Health Organization (WHO) draft plan for containment of wild polioviruses has been published for comment. Commissions and committees for certification of eradication have been established. Still under discussion is the question of the appropriate strategy for stopping oral polio vaccine (OPV) immunization. Studies are underway to determine whether vaccine-derived polioviruses will continue to circulate after OPV cessation and the potential disease consequences of that circulation.

Outbreak of paralytic poliomyelitis in Albania, 1996: high attack rate among adults and apparent interruption of transmission following nationwide mass vaccination.

After >10 years without detection of any cases of wild virus-associated poliomyelitis, a large outbreak of poliomyelitis occurred in Albania in 1996. A total of 138 paralytic cases occurred, of which 16 (12%) were fatal. The outbreak was due to wild poliovirus type 1, isolated from 69 cases. An attack rate of 10 per 100,000 population was observed among adults aged 19-25 years who were born during a time of declining wild poliovirus circulation and had been vaccinated with two doses of monovalent oral poliovirus vaccines (OPVs) that may have been exposed to ambient temperatures for prolonged periods. Control of the epidemic was achieved by two rounds of mass vaccination with trivalent oral poliovirus vaccine targeted to persons aged 0-50 years. This outbreak underscores the ongoing threat of importation of wild poliovirus into European countries, the importance of delivering potent vaccine through an adequate cold chain, and the effectiveness of national OPV mass vaccination campaigns for outbreak control.

Certifying the elimination of poliomyelitis from Europe: advancing towards global eradication.

UNLABELLED: Indigenous wild polioviruses have been virtually eliminated from the 51 countries of the European Region of the World Health Organization (WHO), an achievement that is of critical importance to the global initiative to eradicate poliomyelitis by the year 2000. An international commission has been established to certify the elimination of poliomyelitis from this region. European countries have recently been requested to establish National Certification Committees to review and submit the necessary documentation and surveillance data. In some Western European countries where polio has not been reported for many years, the challenge will be to produce robust evidence demonstrating both the current absence of wild poliovirus and the means to promptly detect and respond to possible importations of wild poliovirus for the next several years, up to global eradication and the cessation of polio vaccination. KEY MESSAGES: 1. laboratory-based surveillance with collection of faecal specimens is necessary to demonstrate the absence of indigenous wild poliovirus 2. certification can only occur after all countries have demonstrated the absence of indigenous wild polioviruses for at least 3 years and have the means to detect and respond to importations of wild poliovirus for several years into the future 3. any single case of poliomyelitis in Europe now requires an immediate public health response which includes virological investigation and prompt notification to the World Health Organization.

Perspectives from the global poliomyelitis eradication initiative.

Ten years after the year 2000 target was set by the World Health Assembly, the global poliomyelitis eradication effort has made significant progress towards that goal. The success of the initiative is built on political commitment within the endemic countries. A partnership of international organizations and donor countries works to support the work of the countries. Interagency coordinating committees are used to ensure that all country needs are met and to avoid duplication of donor effort. Private sector support has greatly expanded the resources available at both the national and international level. At the programmatic level, rapid implementation of surveillance is the key to success, but the difficulty of building effective surveillance programmes is often underestimated. Mass immunization campaigns must be carefully planned with resources mobilized well in advance. Programme strategies should be simple, clear and concise. While improvements in strategy and technology should be continuously sought, changes should be introduced only after careful consideration. Careful consideration should be given in the planning phases of a disease control initiative on how the initiative can be used to support other health initiatives.

The impact of immunization control activities on measles outbreaks in middle and low income countries.

BACKGROUND: The World Health Organization recommended strategy for responding to measles outbreaks in developing countries does not promote the use of immunization campaigns due to their high cost, disruptive nature and limited impact. Given the substantial morbidity and mortality associated with such outbreaks, a literature review was conducted as a basis for re-evaluating this policy. METHODS: Reports of supplementary immunization activities that were performed to control measles outbreaks in middle or low income countries were identified. The impact of the immunization activities on the course of each outbreak was evaluated by examining the data provided. RESULTS: Of 66 reports detailing a measles outbreak in a middle or low income country, 17 described supplementary immunization activities which included seven 'non-selective' immunization campaigns, three 'selective' campaigns and one use of an early 2-dose schedule. Eight of the reports commented on the impact of the response, five of which reported a reduction in outbreak morbidity. Only one of the reports, from an isolated island outbreak, provided sufficient data to support a possible reduction in outbreak-associated morbidity. CONCLUSIONS: There are limited data on the impact of measles outbreak immunization activities from developing countries. The available data do not support a change in the WHO recommended strategy for conducting a limited, if any, immunization response to such outbreaks. Immunization strategies which aim to prevent outbreaks may be more effective than campaigns to interrupt transmission of an outbreak which has already begun.

PIP: Because of their high cost, disruptive nature, and limited impact, immunization campaigns are not recommended by the World Health Organization (WHO) in response to measles outbreaks in developing countries. The authors reviewed the available literature to assess whether that WHO policy should stand or be changed. 66 reports were identified detailing a measles outbreak in middle- or low-income countries. 17 of those reports described supplementary immunization activities to control measles outbreaks which included seven nonselective immunization campaigns, three selective campaigns, and one use of an early two-dose schedule. Eight reports commented upon the impact of the intervention, five of which reported a reduction in outbreak morbidity. Only one report, from an isolated island outbreak, provided sufficient data to support a possible reduction in outbreak-associated morbidity. The available data therefore do not support a change in the WHO-recommended strategy. Immunization strategies to prevent outbreaks may be more effective than campaigns to interrupt transmission of an outbreak which has already begun.