. Special Report Strategies for the Certification of the Eradication of Wild Poliovirus Transmission in the Americas18 Because it appears that the last case of poliomyelitis caused by transmission of indigenous wild poliovirus occurred 2 years ago on 23 August 2992 in Peru, the chalienge for PAHO is to develop a methodology of certification that will convince the rest of the world that the eradication of poliomyelitis in the Americas has been achieved. To that end, the guidelines set forth here allow for a rigorous and stand- ardized evaluation of the certification process by objective experts. As it was during the campaign to eradicate smallpox, epidemiologic surveillance (of acute flaccid paralysis [Al?] in the case of polio) will be the most important component of the certification process. It must be demonstrated that when cases of AFP occur, they will be identified, reported, and investigated in a timely manner to insure that if wild poliovirus is present, it will be identified. INTRODUCTION greater for poliomyelitis than it was for smallpox. On 6 July 1990, delegates to the first Nonetheless, the basic conditions that meeting of the International Certification were essential to certifying the eradica- Commission on Polio Eradication (ICCPE) tion of smallpox appear to be equally ap- established preliminary criteria for certi- plicable to polio eradication: fying that countries in the Americas are free of poliomyelitis (1-2). The Commis- sion recognized that it is extraordinarily no detection of wild poliovirus over difficult to demonstrate with certainty that an extended period of time, in the con- no wild polioviruses are circulating in a text of a surveillance system that is given country, let alone in the Region as good enough to detect both cases and a whole. The challenge is akin to that the virus, should they be present; experienced in certifying that smallpox a thorough country-by-country doc- had been eradicated, but it is even more umentation of program activities and difficult, because the ratio of asympto- findings; and, finally, matic infections to clinical cases is much the judgment by an independent in- ternational commission that suffi- cient evidence is available to support ‘Document prepared by the Expanded Program on Immunization, Pan American Health Organiza- the belief that poliovirus circulation tion, Washington, D.C., June 1993. has ceased. Bulletin of PAHO 27(3), 2993 287 For certification purposes, good sur- for at least 3 years before it could be said veillance is the most important condition with reasonable certainty that eradication that must be met. Surveillance for polio had been achieved. cases encompasses surveillance for all The eventual interruption of indige- conditions causing acute flaccid paralysis nous wild virus transmission is possible (AFP) in persons under 15 years of age, because (1) there is no natural animal res- as well as for suspected polio cases among ervoir for wild poliovirus, (2) infected older persons. It must be demonstrated persons do not excrete the virus for more that when cases of AFP occur, they will than a few weeks, and (3) wild poliovirus be identified, reported, and investigated does not survive in the environment for in a timely manner to ensure that wild prolonged periods of time. Given the large poliovirus will be detected if it is present. number of inapparent infections that ex- The approach to the certification effort ist for every clinical case of the disease must be rigorous yet practical. Ulti- (6-8), measures in addition to case-find- mately, the information gathered must ing will be needed to detect circulating be adequate in quantity and quality to wild polioviruses. Accordingly, wild constitute convincing evidence before the poliovirus surveillance will include test- Commission and the world that trans- ing the stools of normal children and mission of indigenous wild poliovirus has sampling and testing community sewage indeed ceased in the Western Hemi- to help rule out transmission after the last sphere. culture-confirmed paralytic cases have As was the case during the certification been reported. of smallpox eradication, the process of certifying polio eradication will evolve, LESSONS LEARNED FROM requiring refinement at each step (3-4). CERTIFICATION OF SMALLPOX ERADICATION DEFINITION OF POLIOMYELITIS ERADICATION The smallpox eradication certification process in the Americas provided expe- The evolution of the certification pro- rience in four significant areas: (1) the cedures can be understood more readily amount of time that should elapse from by reviewing the definition of and the cri- the last case of paralytic disease to the teria for poliomyelitis eradication as de- point where one could confidently judge veloped by the Directing Council of PAHO that wild virus has ceased circulating, (2) in 1985 (5) and the ICCPE in 1990 (2). the comprehensiveness required of the The definition of polio eradication (as surveillance system, (3) the competence put forth by the Directing Council) is the and diligence needed from the official interruption of transmission of indige- certification committee reviewing na- nous wild poliovirus. Such transmission tional data, and (4) the level of political can and usually does occur as inapparent support that is called for to ensure com- infections. Paralytic cases (one per 100 to pliance with the certification criteria. one per 1 000 infections) serve as a vital First, experience during smallpox erad- indicator of continuing poliovirus spread; ication eventually showed that, with good because of this, their detection is critical. surveillance, no more than a year was At its first meeting in July 1990, members required to confirm that eradication had of the ICCPE decided that the Region been achieved. In practice, however, a should be free of any paralytic polio- 2-year interval was observed. Polio pre- myelitis due to wild poliovirus infection sents a more difficult problem owing to 288 Bulletin of PAHO 27(3), 1993 the occurrence of a large number of in- the first smallpox eradication certification apparent infections, so 3 years after the commission to be established was for last culture-confirmed case has been South America. It suffered from defects provisionally agreed upon as the time in both composition and performance. The period needed for certification of polio commission for smallpox eradication cer- eradication. tification in the Americas had few guide- Preliminary anaIysis of a mathematical lines and its assessment of the situation model developed for statistical validation was brief and not rigorous. Fortunately, of the eradication of indigenous wild subsequent history demonstrated that poliovirus in the Americas (Debanne et smallpox had indeed been eradicated from al., unpublished) indicates that the 3-year South America. This lack of rigor will not period of freedom from paralytic polio- be the case with polio certification be- myelitis may need to be extended to 4 cause the epidemioIogy of polio, i.e., years. The provisional results of this study asymptomatic transmission of the virus, suggest that with good AFP surveillance presents many more challenges to erad- and no detected culture-confirmed polio ication than did smalIpox. cases, the probability that wild poliovirus Finally, interest and efforts invested in would continue to circulate undetected the earlier certification process varied for more than 4 years is less than 5%. greatIy from country to country. Coun- This probability continues to decrease over tries from which smallpox had just been time. eradicated showed great interest in cer- Second, the quality of the surveillance tification, whereas those countries in system that operated during the small- which the disease had been eliminated pox eradication campaign was demon- many years before did not consider cer- strated by the comprehensiveness of the tification a high priority. In the Americas, reports of rash illness with fever and governments will have to mobilize ade- chicken pox cases once smallpox cases quate numbers of staff to prepare prop- had ceased to occur. These reports were erly for Region-wide certification of polio especially helpful in convincing members eradication. of certification committees that if small- Although details of the process may be pox virus had been circulating, it would modified as certification progresses, the have been detected and reported. Sur- essential features-adequate prepara- veillance for AFP serves this role in the tion, detailed documentation of the evi- polio program. dence of at least 3 years of freedom from For polio, as for smallpox, the shorter poliomyelitis, and the independence and the time that has eIapsed since the last authority of the certification team-must case, the more sensitive the surveillance remain unchanged throughout. program must be. As more time elapses, many hundreds of additional infections are needed to sustain chains of trans- CRITERIA FOR CERTIFICATION mission, with higher probabilities that in- OF POLIO ERADICATION fections will be detected if present. Also as with smallpox, areas of greatest inter- The measures to be taken are set forth est are highly populated areas, given that below (I): chains of infection are not readily sus- tained in sparsely populated, remote 1. Verification of the absence of virol- regions. ogidly confirmed indigenous polio- Third, as discussed by Fenner et al. (3), myelitis cases in the Americas for a Special Report 289 period of at least 3 years under cir- However, this does not imply that cer- cumstances of adequate surveillance; tification should wait until global eradi- 2. Verification of the absence of de- cation has been achieved. As with small- tectable wild polioviruses from com- pox, the concept of eradication can apply munities as determined by testing to entire continents, such as the Ameri- of stools from normal children and, cas; but to be certified, each country’s where appropriate, testing of waste- surveillance system must be capable of water from high-risk populations; recognizing and containing importations 3. On-site evaluation by national cer- of wild poliovirus when they occur.