Status of Poliomyelitis Eradication in Europe and the Central Asian Republics of the Former Soviet Union

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1 S76 Status of Poliomyelitis Eradication in Europe and the Central Asian Republics of the Former Soviet Union George Oblapenko and Roland W. Sutter Regional Office for Europe, World Health Organization, Copenhagen, Denmark; National Immunization Program, Centers for Disease Control and Prevention, Atlanta, Georgia In the European Region of the World Health Organization, all countries in which polio is endemic have adopted the following strategies: achievement of high routine vaccination coverage, implementation of supplemental immunization activities; and enhancement of surveillance for poliomyelitis. In 15,205 cases of poliomyelitis were reported. Routine coverage among 1-year-olds with three doses of poliovirus vaccine was 89% in 15. Ten countries conducted national immunization days (NIDs). Twenty-four countries (48%) adopted acute flaccid paralysis (AFP) surveillance. Use of NIDs has decreased poliomyelitis incidence in the seven countries in which polio is endemic (Armenia, Azerbaijan, Kazakhstan, Turkey, Turkmenistan, Tajikistan, Uzbekistan) from 203 cases in 14 to 47 in 15, a 77% reduction. Full implementation of the strategies to achieve eradication in the countries in which polio is endemic, including those countries with epidemic poliovirus transmission during 15, is likely to accomplish regional eradication of poliomyelitis by the year 2000 or earlier. In 1984, the Regional Committee, the governing body ofthe World Health Organization (WHO) Regional Office for Europe, adopted the goal of indigenous poliomyelitis elimination. In 1989, the committee endorsed the plan of action for the eradication of poliomyelitis from the European Region by the year 2000 [1]. This report summarizes the current status ofthe eradication initiative in the European Region and highlights the remaining problems that must be overcome to successfully accomplish the eradication target. Overview Routine vaccination program. The 50 countries ofwho's European Region use different vaccination schedules and vaccines (or combination of vaccines) to prevent poliomyelitis [2]. The majority ofthese countries rely on oral poliovirus vaccine (OPV). Several western European countries use exclusively inactivated poliovirus vaccine (IPV), including Finland, France, Netherlands, Norway, and Sweden, while Denmark, Hungary, and Estonia adopted sequential schedules of IPV followed by OPV. Israel uses a sequential schedule of OPV followed by IPV [3]. All countries in the region recommend vaccination schedules that include at least three doses of poliovirus vaccines to be administered in the first year of life and reinforcing or booster doses to be given during the second year of life [2]. Several countries recommend supplemental doses of poliovirus vaccine at school entry and later in life. Routine vaccination coverage with three doses of poliovirus vaccine increased from 82%-84% in to 88% in 14 Reprints or correspondence: Dr. G. Oblapenko, 8 Scherfigsvey, DK-2100, Copenhagen, Denmark. The Journal oflnfectious Diseases 17; 175(Suppl 1):S by The University of Chicago. All rights reserved /97/75S1-0014$01.00 and 89% in 15. In 12-13, vaccine shortages affected certain countries of the New Independent States (NIS) of the former Soviet Union or regions within these countries. The vaccine supply problems in the NIS have since been resolved, and vaccination coverage has returned to pre-independence levels among the NIS. Although routine vaccination coverage in the region was 89% in 15, substantial differences in poliovirus vaccine coverage continue to persist among countries and among geopolitical units within countries. For example, the routine coverage in Bosnia and Herzegovina was 45% in 13 and 69% in 15. During 11-13, the European Region asked member countriestomonitorand report vaccinationcoverage by geopolitical units (districts or oblasts/provinces). On the basis of these data, 83% ofthe districts achieved vaccination coverage of ~80% during 11, and > 90% of the districts achieved this level ofcoverage each year during Surveillance. Surveillance continues to be strengthened, and the number of countries in the region conducting acute flaccid paralysis (AFP) surveillance has increased from 5 countries in 11 to 14 countries in 12, 15 countries in 13, 17 countries in 14, and 24 countries in 15. AFP surveillance has been adopted in the following countries: Albania, Armenia, Azerbaijan, Belarus, Bulgaria, Croatia, Czech Republic, Finland, Hungary, Ireland, Italy, Kazakhstan, Kyrgyzstan, Moldova, Netherlands, Poland, Romania, Russia, Slovak Republic, Slovenia, Turkey, Turkmenistan, Ukraine, and United Kingdom. Surveillance indicators are being increasingly reported, particularly the proportion of cases detected within 7 days of onset, the proportion of cases for which 2 stool specimens were collected, and the rate of AFP in children ::::; 15 years of age. Close monitoring ofthese indicators and maturation ofthe AFP surveillance system should rapidly increase the completeness and quality of AFP surveillance in the European Region. To assist countries in improving surveillance, the regional office is conducting formal surveillance assessments for vac-

2 JID 17;175 (Suppll) Status of Polio Eradication in Europe S til 500 CD til ns o 400 '0...! 300 E :::s Z I NIS 0 Turkey ~ Rest I Figure 1. Reported poliomyelitis cases, by area, European Region ofthe World Health Organization, NIS = New Independent States of the former Soviet Union; Rest = countries of the European Region ofthe World Health Organization with the exception ofnis and Turkey. cine-preventable disease reporting, with emphasis on poliomyelitis and diphtheria surveillance. To date, surveillance assessments have been conducted in Hungary (14) and in the Russian Federation (15). Increasing demand for these assessments suggests that many countries ofthe region place a high priority on improving surveillance. Secular trends. In 15, the 50 countries of the European Region of WHO (which includes the Central Asian Republics ofthe former Soviet Union) reported 205 cases ofpoliomyelitis (presumed or known to be due to wild poliovirus). During the previous 5 years (10-14), 373, 318, 181, 198, and 216 indigenously acquired cases ofpoliomyelitis were reported in the European Region, a substantial decrease from the number of poliomyelitis cases reported in the early 1980s (figure 1). Poliomyelitis presumed to be due to wild poliovirus was reported from 11 (22%) of the 50 countries of the European Region in 14. With the exception of Turkey and Greece, all of these countries are located in eastern Europe (Belarus, Romania, and Russia), the Transcaucasus region (Armenia and Azerbaijan), or central Asia (Kazakhstan, Tajikistan, Turkmenistan, and Uzbekistan). Four countries (Azerbaijan, Turkey, Tajikistan, and Uzbekistan) have consistently reported >5 cases of poliomyelitis during each year from 11 to 14. In 15, >75% of the cases were reported from the Russian Federation (figure 2). In addition to other NIS countries and Turkey, Yugoslavia (Serbia and Montenegro) reported 3 cases. In 1980, about one-third of the poliomyelitis cases were reported from the Soviet Union, one-third from Turkey, and onethird from the remaining European countries. However, by 14 and 15, the vast majority of cases (85% and 84%, respectively) were reported from the NIS of the former Soviet Union (figure 2). Following the implementation of supplemental immunization activities (especially national immunization days [NIDs]) in 14 in Uzbekistan and 15 in all countries of the region in which polio is endemic (Armenia, Azerbaijan, Kazakhstan, Tajikistan, Turkey, Turkmenistan, and Uzbekistan), a substantial decrease in reported cases of poliomyelitis was documented. During 14, 203 cases of poliomyelitis were reported from these 7 countries. During 15, only 47 cases of poliomyelitis were reported from these countries, for a 77% decrease in poliomyelitis incidence. In 14, Uzbekistan reported 54% (117/216) ofregional poliomyelitis cases. After NIDs in 14 (October and November) and 15 (April and May), Uzbekistan reported only one case ofpoliomyelitis in 15 (figure 3). In countries conducting NIDs in 15, 90% ( ) of reported cases in 14 were in children <4 years of age, 7% (151201) were in children 4-10 years of age, and 3% (8/201) were in persons of unknown ages. The Russian Federation reported an outbreak of 154 poliomyelitis cases in 15, primarily from Chechnya (a region located in the North Caucasus). After an absence ofimmunization activities during the preceding 3 years because of the civil war, poliomyelitis cases were reported between June and October from Chechnya (146 cases), from a neighboring area (Inguschetya, 4 cases), and from other oblasts (4 cases). Extensive control efforts with two rounds of OPV covering >60% (round 1) and 89% (round 2) of children <5 years of age appear to have terminated the outbreak. There has been a substantial decrease in number ofgeopolitical units (districts or oblasts/provinces) in which poliovirus circulated during the last 3 years, from 105 (14 countries) in 12 to 64 (12 countries) in 13, 73 (11 countries) in 14, and 40 (9 countries) in 15. Supplemental immunization. On 7 April 15, the world celebrated World Health Day, which was devoted to global polio eradication and the theme "Target 2000-A World Without Polio" [4]. To accelerate polio eradication efforts, 18 geographically contiguous countries in Europe, central and south Asia, and the Middle East cooperated in conducting coordinated NIDs with OPV to coincide with World Health Day activities [5] (figure 4). This effort was designated "Operation MECACAR" (for Mediterranean, Caucasus, and Central Asian Republics) [5]. To maximize the geographic area covered and the number of children targeted simultaneously for mass vaccination campaigns with OPV, adjoining countries located in Europe (Armenia, Azerbaijan, Bulgaria, Georgia, Turkey), central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan), southern Asia (Afghanistan, Iran, Pakistan), and the Middle East (Iraq, Jordan, Lebanon, Syria, Gaza, Jericho, and the West Bank) carried out synchronized NIDs. A total of 64 million children <5 years old were targeted to receive two doses each of OPV (table 1). Participating countries in each region have provisionally reported a high proportion of the total polio cases in their respective regions in 14 (European Region, 201/216 [93%]; Eastern Mediterranean Region, [68%]). Many ofthe these countries have previously con-

3 S78 Oblapenko and Sutter JID 17;175 (Suppl 1) tn 200 =150 ca o '0 100 ~ CJ).Q E 50 ::J Z Figure 2. Reported poliomyelitis cases, New Independent States ofthe former Soviet Union, ducted NIDs, including Azerbaijan (13), Lebanon (14), Iran (14), Syria (13 and 14), Pakistan (14), and Uzbekistan (14); many of the other countries have had experience with conducting subnational immunization days (SNIDs). Based on the desirability of scheduling mass vaccination campaigns during the low polio incidence season, either the first round (European Region) or the second round (Eastern Mediterranean Region) of NIDs has been scheduled on or around April 7. With the exception of Bulgaria, countries participating in this effort planned to repeat NIDs in 16 and 17. Reported coverage levels with OPV during each round of NIDs in the 10 participating countries of Europe and Central Asian Republics of the former Soviet Union were 89% in Bulgaria to % in Kazakhstan, Kyrgyzstan, Tajikistan, and Turkmenistan during the first round and 84% in Bulgaria to % in Kazakhstan, Kyrgyzstan, and Turkmenistan during the second round (table 2). Ofthe 10 countries from the European Region participating with these synchronized NIDs, 9 (90%) achieved coverage rates >90% in both NID rounds. 50 III 3l ~ 30 '0.. CD ~ 20 ~ Z 40 NIDs NIDs n o ~"-.-J...L...II" Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Figure 3. Reported poliomyelitis cases, by month, Uzbekistan, NIDs, national immunization days. Several countries, including Bosnia and Herzegovina, Moldova, Russian Federation, and Ukraine, have conducted or are planning to conduct NIDs in 16, while Romania and Yugoslavia (Serbia and Montenegro) have conducted or are planning to conduct SNIDs in 16. Laboratory network. To ensure that adequate laboratory support is available to countries in the European Region, a poliovirus laboratory network was created in 11. Four regional laboratories (National Public Health Institute, Helsinki; Institut Pasteur, Paris; National Institute of Public Health and Environmental Protection, Bilthoven, Netherlands; and Institute of Poliomyelitis and Viral Encephalitis, Moscow) each support 8-12 countries. A fifth regional laboratory (National Public Health Laboratory, London), as well as the regional laboratories in Helsinki, Bilthoven, and Paris, also provides specialized support to the global laboratory network. Two subregional laboratories provide support to selected countries: Istituto Superiore di Sanita, Rome, supports Albania and Malta, and the National Enterovirus Laboratory, Sofia, Bulgaria, supports the former Yugoslav republic ofmacedonia. The priority for the laboratory network is to improve the performance of the 41 national laboratories to permit WHO accreditation for polio eradication. Since 12, three training courses have been conducted with participation from 26 countries. Proficiency testing has identified several laboratories in need of further assistance. One benefit of the collaboration between national and regionallaboratories is the molecular determination ofcirculating poliovirus strains [6-8]. The epidemiologic and virologic aspects ofthe poliomyelitis outbreaks occurring in the European Region and elsewhere between 1976 and 15 were summarized recently [9]. The outbreak of poliomyelitis in Tajikistan in 11 was due to a genotype of poliovirus type 1 closely related to progeny virus isolated previously in the Indian subcontinent [7]. Subsequently, the same genotype of poliovirus type 1 was associated with epidemic transmission in the

4 JID 17;175 (Suppll) Status of Polio Eradication in Europe S79 Figure 4. Countries conducting synchronized national immunization days, European and Eastern Mediterranean regions of World Health Organization, 15. Ukraine in 12-13, Uzbekistan in 14, and Chechnya in 15 [8] (Lipskaya G, Moscow State University, personal communication, 15). The outbreak of poliomyelitis in Uzbekistan during 13 represents poliovirus type 3 previously isolated in other areas of the former Soviet Union and, thus, may be a genotype indigenous to the former Soviet Union (Lipskaya G, Moscow State University, personal communication, 15). Continued analyses of poliovirus isolates in the European Region will allow documentation of the molecular Table 1. Features of the coordinated national immunization days (NIDs) for polio eradication, European and Eastern Mediterranean Regions, 15. Eastern European Mediterranean Region* Region' Total epidemiology of indigenous poliovirus genotypes, the circulation of imported strains, and the association ofvaccine-related poliovirus with paralytic cases (i.e., vaccine-associated paralytic poliomyelitis). Remaining obstacles. The progress toward poliomyelitis eradication in the European Region was achieved despite civil unrest or war, which has affected at least some countries (including the former Yugoslavia, Azerbaijan, Armenia, Chechnya, and Tajikistan), and the recent large-scale reemergence of diphtheria in the NIS countries [10-12]. Lack of financial support for vaccine purchases caused major shortages of OPV and other vaccines in some countries during The Table 2. Dates and reported vaccination coverage achieved during each round of national immunization days, by country, European Region, 15. Countries participating Target population <5 years old (millions) Doses ofopv required (millions) Dates of NIDs (no. of countries') March April May NOTE. OPV, oral poliovirus vaccine. * Armenia, Azerbaijan, Bulgaria, Georgia, Kazakhstan, Kyrgyzstan, Tajikistan, Turkey, Turkmenistan, and Uzbekistan. t Afghanistan, Iran, Iraq, Jordan, Lebanon, Pakistan, Syria, Gaza, Jericho, and West Bank. t Each country conducted 2 rounds of NIDs. Country Armenia Azerbaijan Bulgaria Georgia Kazakhstan Kyrgyzstan Tajikistan Turkey Turkmenistan Uzbekistan Dates (month/days) Reported OPV coverage (%) First round Second round First round Second round 3/24-7 3/3-7 4/ / / /3-7 5/ /

5 S80 Oblapenko and Sutter JID 17;175 (Suppl I) outbreak in Azerbaijan probably escalated due to the lack of OPV from March 13 until November 13. Similarly, the outbreak in Uzbekistan during can be attributed to major shortages of OPV [13]. The outbreak in Chechnya in 15 also appears to have been facilitated by the discontinuation of vaccination during the previous 3 years. Although OPV shortages contributed to the occurrence of epidemic poliomyelitis in some countries, other countries with sufficient OPV continue to experience endemic poliovirus transmission. For example, Turkey annually reported between 14 and 29 cases ofpoliomyelitis during Low coverage ofchildren < 1 year ofage in Turkey through the routine vaccination program (69% with three doses of OPV in 12, 76% in 13, and 86% in 15) may be the major factor responsible for the ongoing endemic transmission of poliomyelitis. Other countries with ongoing endemic poliovirus transmission and relatively low vaccination coverage levels during 12 include Azerbaijan (70%; 13 coverage decreased to 40%; 94% in 14), Tajikistan (unknown; 13 coverage increased to 76%; 92% in 14), Uzbekistan (85%; 13 coverage decreased to 49%; 79% in 14), and Russia (69%; 13 coverage increased to 82%; 88% in 14). The coordinated and synchronized NIDs conducted during 15 in the European Region were successful in achieving high coverage levels in the targeted age groups. Future gains toward consolidating the progress in interrupting wild poliovirus transmission will depend on repeating the NIDs during 16 and 17. In addition, the commitments of Bosnia and Herzegovina, Moldova, Russian Federation, and Ukraine to conduct NIDs in 16 will be critical in eliminating wild poliovirus reservoirs and improving population immunity against poliomyelitis in these countries. Comment Poliomyelitis eradication is rapidly becoming a reality in some areas ofthe world [14]. Achievement ofregional elimination of poliomyelitis in the Western Hemisphere [15, 16] and western Europe [1], progress toward elimination in countries of east Asia [14], including China [17], the Philippines [18], and Vietnam [19], and the development of polio-free zones in southern Africa [20] and North Africa and the Arabian Peninsula [21] are encouraging signs ofprogress. However, increased efforts are needed in other areas, including the Indian subcontinent, which conducted NIDs for the first time in December 15 and January 16 [22], sub-saharan Africa, and in the remaining countries of the European Region in which polio is endemic (i.e., Transcaucasus and Central Asian Republics of the former Soviet Union) [1]. Many ofthe poliovirus genotypes responsible for recent epidemics in Europe (including the outbreak of 71 cases due to poliovirus type 3 in the Netherlands in 12-13) probably originated from the Indian subcontinent [7, 8, 23]. Progress toward poliomyelitis eradication in the Indian subcontinent will be critical to maintaining poliofree status in many countries of Europe and elsewhere. The European countries can be divided into three major epidemiologic blocks: the northern and western European countries, which have achieved high vaccination coverage and eliminated poliomyelitis as an indigenous disease more than a decade ago but are subject to importations of poliovirus, particularly among groups objecting to vaccination (e.g., as occurred in the Netherlands in [24]) or groups with suboptimal coverage (e.g., traveling people or gypsies) [25]; Balkan and Asia Minor countries, which, with the exception of Turkey, controlled poliomyelitis well over the past 2 decades, interrupted by small outbreaks, the most recent of which occurred in in Bulgaria [25] and Romania [26]; and the former Soviet Union, in which two major reservoirs of poliovirus have emerged, the Caucasus and the republics of Central Asia. Increased efforts to accomplish regional elimination ofpoliomyelitis in Europe must be targeted to these poliovirus reservoirs and the remaining countries in which polio is endemic. Routine vaccination coverage must be increased to >90% among children < 1 year of age with at least three doses ofopv in all geopolitical units, and supplemental OPV vaccination activities, including NIDs, must be continued by the countries in which polio is endemic. NIDs must reach hard-to-reach populations, including gypsies and refugees. Both the NIDs in Bulgaria in 15 and the SNIDs in Romania in 16 have made special efforts to include gypsy populations. Only 40 geopolitical units (l%) reported cases ofpoliomyelitis in the European Region during 15. Thus, focused houseto-house iml11unization activities (a WHO strategy known as "mopping up") to reach infants and children not routinely covered by immunization programs are becoming a feasible strategy to be implemented in high-risk geopolitical units starting in 16. The collaboration among the 18 countries and national identities in the European and Eastern Mediterranean regions to conduct synchronized NIDs for polio eradication is remarkable since it has brought together countries with vastly different political systems, economic organizations, racial and ethnic groups, and religions. Negotiations were necessary to curtail hostilities and secure cease-fires during the vaccination campaigns in countries with internal conflict or civil wars; similar arrangements had been mediated in countries of the Americas [15] and more recently in the Philippines [18] to ensure the success of NIDs. Reported poliomyelitis incidence in the European Region of WHO has reached a plateau in the 10s, with cases reported each year. Further reductions in poliomyelitis incidence and the elimination of poliovirus from the remaining countries in which polio is endemic cannot be achieved through routine childhood vaccination alone but require supplementary immunization activities, including NIDs. In the Eastern Mediterranean Region of WHO, substantial decreases in polio inci-

6 JID 17; 175 (Suppl 1) Status of Polio Eradication in Europe S81 dence were noted during the past 5 years. Nonetheless, nearly 2500 cases of polio were reported in 13, and 738 cases were reported in 15. The unique cooperation of countries to conduct synchronized NIDs and the vaccination coverage rates achieved (i.e., vaccination coverage of >90% of the target population in 90% of participating countries in the European Region) have already led to substantial decreases in the incidence of poliomyelitis in the participating countries. NIDs will be repeated in 16 and 17; these efforts hold promise that the entire region will be brought to the threshold of elimination of poliovirus within the next 2-3 years. Full implementation of additional immunization strategies, including mopping up, together with successful procurement of all needed vaccines, heightened political commitment, improved surveillance, and a functioning laboratory network, will be critical to accomplish the eradication of poliomyelitis from the European Region before the target year of References 1. Centers for Disease Control and Prevention. Status of poliomyelitis eradication-europe and the Central Asian Republics, 13. MMWR Morb Mortal Wkly Rep 14;43: Expanded Programme on Immunization. Overview of immunization programmes in the European region-14/15. Copenhagen: World Health Organization, Regional Office for Europe, Slater PE. Postoutbreak vaccination policy in Israel. Am J Dis Child 10; 144: Centers for Disease Control and Prevention. World Health Day-April 7, 15. Global poliomyelitis eradication. MMWR Morb Mortal Wkly Rep 15;44: Centers for Disease Control and Prevention. Mass vaccination with oral poliovirus vaccine-asia and Europe, 15. MMWR Morb Mortal Wkly Rep 15;44: Rico-Hesse R, Pallansch MA, Nottay BK, Kew OM. Geographic distribution of wild poliovirus type 1 genotypes. Virology 1987; 160: Lipskaya GY, Chervonskaya EA, Belova GI, et al. Geographical genotypes (geotypes) of poliovirus case isolates from the former Soviet Union: relatedness to other known poliovirus genotypes. J Gen Virol 15; 76: Kew OM, Mulders MN, Lipskaya GY, da Silva EE, Pallansch MA. Molecular epidemiology of po1ioviruses. Semin Viro1 15;6: Patriarca PA, Sutter RW, Oostvogel PM. Outbreaks of paralytic poliomyelitis, J Infect Dis 17;175(suppll):S Centers for Disease Control and Prevention. Diphtheria outbreak-russian Federation, MMWR Morb Mortal Wkly Rep 13; 42:840-1, Centers for Disease Control and Prevention. Diphtheria epidemic- New Independent States of the fonner Soviet Union, MMWR Morb Mortal Wkly Rep 15;44: Hardy IRB, Dittmann S, Sutter RW. Current situation and control strategies of resurgence of diphtheria in the newly independent states of the former Soviet Union. Lancet 16;347: Sutter RW, Chudaiberdiev YK, Vaphakulov SH, Tursunova D, Oblapenko G, Iskandarov Tl. A large outbreak of poliomyelitis following temporary cessation of vaccination in Samarkand, Uzbekistan, J Infect Dis 17; 175(suppl 1):S Centers for Disease Control and Prevention. Progress toward global poliomyelitis eradication, MMWR Morb Mortal Wkly Rep 15;44:273-5, de Quadros CA, Andrus JK, Olive IM, de Macedo CG. Polio eradication from the Western hemisphere. Annu Rev Public Health 12; 13: Centers for Disease Control and Prevention. Certification of poliomyelitis eradication-the Americas, 14. MMWR Morb Mortal Wkly Rep 14;43: Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication-china, MMWR Morb Mortal Wkly Rep 14;43: Centers for Disease Control and Prevention. National polio immunization days and status of polio eradication-philippines, 13. MMWR Morb Mortal Wkly Rep 14;43:6-7, Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication- Socialist Republic of Vietnam, MMWR Morb Mortal Wkly Rep 14;43; Centers for Disease Control and Prevention. Emerging polio-free zonesouthern Africa, MMWR Morb Mortal Wkly Rep 14; 43: Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication-eastern Mediterranean Region, MMWR Morb Mortal Wkly Rep 15;43:809-11, Centers for Disease Control and Prevention. Progress toward poliomyelitis eradication-india, December 15 and January 16. Morb Mortal Wkly Rep 16;45: Oostvogel PM, van Wijngaarden JK, van der Avoort HGAM, et al. Poliomyelitis in an unvaccinated community in the Netherlands, Lancet 14; 344: Bernal A, Garcia-Saiz A, Liacerc A, de Ory F, Pello 0, Najera R. Poliomyelitis in Spain, : virologic and epidemiologic studies. Am J Epidemio1 1987; 126: World Health Organization, Expanded Programme on Immunization. Poliomyelitis outbreak, Bulgaria. Wkly Epidemiol Rec 12;67: Strebel PM, Aubert-Cambiescu A, Ion-Nedelcu N, et al. Paralytic poliomyelitis in Romania, : evidence for a high risk of vaccineassociated disease and reintroduction of wild-virus infection. Am J Epidemioll4; 140: